Access To a Variable of another script which in another scene

Hey There iam trying to get a variable from another script which is in another scene

The Goal here that there is 2 scenes the first scene there is a Money Value which is named AmericaFire and the script that include the money value is attached to the main camera and this is the code:

 public int AmericaFire;
     //Tank
     public int Tank_Amount;
     public Texture Tank;
     //F16
     public int F16_Amount;
     public Texture F16;
     //Rafael
     public int Rafael_Amount;
     public Texture Rafael;
     //LongMissile
     public int LongMissile_Amount;
     public Texture Long_Missile;
     //AntiPlaneMissile
     public int AntiPlaneMissile_Amount;
     public Texture AntiPlane_Missile;
     //ReflecteHit
     public int ReflecteHit_Amount;
     public Texture Reflecte_Hit;
     //DefendHit
     public int DefendHit_Amount;
     public Texture Defend_Hit;
     
     // Use this for initialization
     void Start () {
         //AmericaFire
         PlayerPrefs.GetInt ("AmericaFirePref");
         AmericaFire = PlayerPrefs.GetInt ("AmericaFirePref");
         //AmericaTank
         PlayerPrefs.GetInt ("AmericaTankPref");
         Tank_Amount = PlayerPrefs.GetInt ("AmericaTankPref");
         //AmericaF16
         PlayerPrefs.GetInt ("AmericaF16Pref");
         F16_Amount = PlayerPrefs.GetInt ("AmericaF16Pref");
         //AmericaRafael
         PlayerPrefs.GetInt ("AmericaRafaelPref");
         Rafael_Amount = PlayerPrefs.GetInt ("AmericaRafaelPref");
         //AmericaLongMissile
         PlayerPrefs.GetInt ("AmericaLongMissilePref");
         LongMissile_Amount = PlayerPrefs.GetInt ("AmericaLongMissilePref");
         //AmericaAntiPlaneMissile
         PlayerPrefs.GetInt ("AmericaAntiPlaneMissilePref");
         AntiPlaneMissile_Amount = PlayerPrefs.GetInt ("AmericaAntiPlaneMissilePref");
         //AmericaReflecteHit
         PlayerPrefs.GetInt ("AmericaReflecteHitPref");
         ReflecteHit_Amount = PlayerPrefs.GetInt ("AmericaReflectePref");
         //AmericaDefendHit
         PlayerPrefs.GetInt ("AmericaDefendHitPref");
         DefendHit_Amount = PlayerPrefs.GetInt ("AmericaDefendHitPref");
 
 
     }
     
     
     // Update is called once per frame
     void Update () {
         //AmericaFire
         PlayerPrefs.SetInt ("AmericaFirePref", AmericaFire);
         //AmericaF16
         PlayerPrefs.SetInt ("AmericaF16Pref", F16_Amount);
         //AmericaRafael
         PlayerPrefs.SetInt ("AmericaRafaelPref", Rafael_Amount);
         //AmericaLongMissile
         PlayerPrefs.SetInt ("AmericaLongMissilePref", LongMissile_Amount);
         //AmericaAntiPlaneMissile
         PlayerPrefs.SetInt ("AmericaAntiPlaneMissilePref", AntiPlaneMissile_Amount);
         //AmericaReflecteHit
         PlayerPrefs.SetInt ("AmericaReflecteHitPref", ReflecteHit_Amount);
         //AmericaDefendHit
         PlayerPrefs.SetInt ("AmericaDefendHitPref", DefendHit_Amount);
         //AmericaTank
         PlayerPrefs.SetInt ("AmericaTankPref", Tank_Amount);
 
     }
     
     void OnGUI () {
         //Back
         if (GUI.Button (new Rect (1225, 0, 50, 40), "Back")) {
             Application.LoadLevel("Countres_Screen");
         }
         
         //Attack
         if (GUI.Button (new Rect (0, 0, 80, 80), "Attack")) {
             
             Application.LoadLevel("AmericaWar");
         }
         //Fire
         GUI.Label (new Rect (390, 250, 120, 120), "Fire:");
         GUI.Label (new Rect (390, 270, 120, 120), "" + AmericaFire);
         
         //Tank
         GUI.DrawTexture (new Rect (500, 250, 150, 120), Tank);
         GUI.TextArea (new Rect (650, 250, 150, 40), "Tank it's a War Vehicle armed with HeavyGun");
         GUI.Label (new Rect (650, 290, 120, 120), "Price: 300,000F");
         GUI.Label (new Rect (650, 320, 120, 120), "haggling: 30,000F");
         GUI.Label (new Rect (650, 305, 120, 120), "Damage: 60,000F");
         GUI.Label (new Rect (570, 370, 120, 120), "Amount:");
         GUI.Label (new Rect (620, 370, 120, 120), "" + Tank_Amount);
         if (GUI.Button (new Rect (650, 340, 50, 40), "Buy")) {
             if(AmericaFire < 300000) {
             }
             else if(AmericaFire >= 300000) {
                 AmericaFire -= 300000;
                 Tank_Amount += 1;
             }
         }
         if (GUI.Button (new Rect (700, 340, 50, 40), "Sell")) {
             if(Tank_Amount <= 0) {
             }
             if(Tank_Amount > 0) {
                 Tank_Amount -= 1;
                 AmericaFire += 300000;
             }
         }
         
         //F16
         GUI.DrawTexture (new Rect (500, 460, 150, 120), F16);
         GUI.TextArea (new Rect (650, 435, 150, 60), "F16 it's a powerful WarShip it can deal massive damage");
         GUI.Label (new Rect (650, 500, 180, 120), "Price: 4,500,000F");
         GUI.Label (new Rect (650, 530, 180, 120), "haggling: 2,000,000F");
         GUI.Label (new Rect (650, 515, 180, 120), "Damage: 3,000,000F");
         GUI.Label (new Rect (570, 580, 120, 120), "Amount:");
         GUI.Label (new Rect (620, 580, 120, 120), "" + F16_Amount);
         if (GUI.Button (new Rect (650, 550, 50, 40), "Buy")) {
             if(AmericaFire < 4500000) {
             }
             else if(AmericaFire >= 4500000) {
                 AmericaFire -= 4500000;
                 F16_Amount += 1;
             }
         }
         if (GUI.Button (new Rect (700, 550, 50, 40), "Sell")) {
             if(F16_Amount <= 0) {
             }
             if(F16_Amount > 0) {
                 F16_Amount -= 1;
                 AmericaFire += 4500000;
             }
         }
         
         //Rafael
         GUI.DrawTexture (new Rect (850, 460, 150, 120), Rafael);
         GUI.TextArea (new Rect (1000, 435, 150, 65), "Rafael this WarShip is super power armed with powerful missiles");
         GUI.Label (new Rect (1000, 500, 180, 120), "Price: 3,000,000F");
         GUI.Label (new Rect (1000, 530, 180, 120), "haggling: 300,000F");
         GUI.Label (new Rect (1000, 515, 180, 120), "Damage: 600,000F");
         GUI.Label (new Rect (920, 580, 120, 120), "Amount:");
         GUI.Label (new Rect (970, 580, 120, 120), "" + Rafael_Amount);
         if (GUI.Button (new Rect (1000, 550, 50, 40), "Buy")) {
             if(AmericaFire < 3000000) {
             }
             else if(AmericaFire >= 3000000) {
                 AmericaFire -= 3000000;
                 Rafael_Amount += 1;
             }
         }
         if (GUI.Button (new Rect (1050, 550, 50, 40), "Sell")) {
             if(Rafael_Amount <= 0) {
             }
             if(Rafael_Amount > 0) {
                 Rafael_Amount -= 1;
                 AmericaFire += 3000000;
             }
         }
         
         //LongMissiles
         GUI.DrawTexture (new Rect (850, 250, 150, 120), Long_Missile);
         GUI.TextArea (new Rect (1000, 250, 170, 40), "LongMissiles consider from the most powerful missiles");
         GUI.Label (new Rect (1000, 290, 120, 120), "Price: 1,000,000F");
         GUI.Label (new Rect (1000, 320, 120, 120), "haggling: 100,000F");
         GUI.Label (new Rect (1000, 305, 120, 120), "Damage: 200,000F");
         GUI.Label (new Rect (920, 370, 120, 120), "Amount:");
         GUI.Label (new Rect (970, 370, 120, 120), "" + LongMissile_Amount);
         if (GUI.Button (new Rect (1000, 340, 50, 40), "Buy")) {
             if(AmericaFire < 1000000) {
             }
             else if(AmericaFire >= 1000000) {
                 AmericaFire -= 1000000;
                 LongMissile_Amount += 1;
             }
         }
         if (GUI.Button (new Rect (1050, 340, 50, 40), "Sell")) {
             if(LongMissile_Amount <= 0) {
             }
             if(LongMissile_Amount > 0) {
                 LongMissile_Amount -= 1;
                 AmericaFire += 1000000;
             }
         }
         
         //AntiPlaneMissile
         GUI.DrawTexture (new Rect (850, 70, 150, 120), AntiPlane_Missile);
         GUI.TextArea (new Rect (1000, 70, 280, 40), "AntiPlaneMissile 3 of them can breakdown F16 and 5 of them can breakdown Rafael");
         GUI.Label (new Rect (1000, 110, 120, 120), "Price: 500,000F");
         GUI.Label (new Rect (920, 190, 120, 120), "Amount:");
         GUI.Label (new Rect (970, 190, 120, 120), "" + AntiPlaneMissile_Amount);
         if (GUI.Button (new Rect (1000, 135, 50, 40), "Buy")) {
             if(AmericaFire < 500000) {
             }
             else if(AmericaFire >= 500000) {
                 AmericaFire -= 500000;
                 AntiPlaneMissile_Amount += 1;
             }
         }
         if (GUI.Button (new Rect (1050, 135, 50, 40), "Sell")) {
             if(AntiPlaneMissile_Amount <= 0) {
             }
             if(AntiPlaneMissile_Amount > 0) {
                 AntiPlaneMissile_Amount -= 1;
                 AmericaFire += 500000;
             }
         }
         
         //DefendHit
         GUI.DrawTexture (new Rect (500, 70, 150, 120), Defend_Hit);
         GUI.TextArea (new Rect (650, 70, 200, 40), "Defend Hit one of this can cancel an attack and destroy the weapon");
         GUI.Label (new Rect (650, 110, 120, 120), "Price: 2,000,000F");
         GUI.Label (new Rect (570, 190, 120, 120), "Amount:");
         GUI.Label (new Rect (620, 190, 120, 120), "" + DefendHit_Amount);
         if (GUI.Button (new Rect (650, 135, 50, 40), "Buy")) {
             if(AmericaFire < 2000000) {
             }
             else if(AmericaFire >= 2000000) {
                 AmericaFire -= 2000000;
                 DefendHit_Amount += 1;
             }
         }
         if (GUI.Button (new Rect (700, 135, 50, 40), "Sell")) {
             if(DefendHit_Amount <= 0) {
             }
             if(DefendHit_Amount > 0) {
                 DefendHit_Amount -= 1;
                 AmericaFire += 2000000;
             }
         }
         
         //ReflecteHit
         GUI.DrawTexture (new Rect (0, 250, 150, 120), Reflecte_Hit);
         GUI.TextArea (new Rect (150, 250, 200, 60), "Reflecte Hit one of this can Reflecte the attack to the other team and deal the same damage");
         GUI.Label (new Rect (150, 310, 120, 120), "Price: 3,500,000F");
         GUI.Label (new Rect (40, 370, 120, 120), "Amount:");
         GUI.Label (new Rect (90, 370, 120, 120), "" + ReflecteHit_Amount);
         if (GUI.Button (new Rect (150, 330, 50, 40), "Buy")) {
             if(AmericaFire < 3500000) {
             }
             else if(AmericaFire >= 3500000) {
                 AmericaFire -= 3500000;
                 ReflecteHit_Amount += 1;
             }
         }
         if (GUI.Button (new Rect (200, 330, 50, 40), "Sell")) {
             if(ReflecteHit_Amount <= 0) {
             }
             if(ReflecteHit_Amount > 0) {
                 ReflecteHit_Amount -= 1;
                 AmericaFire += 3500000;
             }
         }
         
         
         
         
     }

and in the another scene there is a button called attack and the use for the button is when it clicked the money value in the another script which called AmericaFire decreases by 60000 and this is the Code for the second script in the second scene:

 void Start () {
 
         GameObject MainCamera = GameObject.Find("MainCamera");
         America America = MainCamera.GetComponent<America>();
         America.AmericaFire -= 60000;
 
     
     }
     
     // Update is called once per frame
     void Update () {
 
     
     }
 
     void OnGUI () {
 
     }

and when i press the button it do nothing and get an error in the console says:

 NullReferenceException: object Reference not set to an instance of an object

what should i do to make it all works and what is the problem here

Note:I tried to make the DontDestroyOnLoad thing and it didnt work

Thank You previously