Quit alert dialog box Android

i need help quickly pleaseee.. i want put the dialog box "are you sure want to quit?" "yes" or "no" in my game 2d because when i touch the exit button, dialog didnt show.

this is my script..

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class uiManager : MonoBehaviour {
     
     public Button[] buttons;
     public audioManager am;
     bool gameOver;
 
    
 
     // Use this for initialization
     void Start () {
 
         
 
     }
     
     // Update is called once per frame
     void Update () {
 
     }
 
     public void gameOverActivated(){
         gameOver = true;
         foreach (Button button in buttons) {
             button.gameObject.SetActive(true);
 
             
 
         }
     }
 
     public void Play(){
         Application.LoadLevel ("level1");
     }
 
 
 
 
     public void Menu(){
 
         Application.LoadLevel ("menucover");
         
 
     }
 
 
     public void HowToPlay (){
 
         Application.LoadLevel ("howtoplay");
     }
 
     public void Exit(){
         Application.Quit ();
 
     }
 
 
 
 }