counting down a variable issue

hi complete noob here lets get to it, my player collects coins. At the end of the level it displays how many coins were collected. The number of coins will be added to the total score. An animation of the numbers decreasing in the text ui box is what i want, ive achieved this from this question here however for some reason it shows the amount of coins that were collected just fine and then counts down to -114 is there something im missing?

here is my script updating the text

 [SerializeField]
 Text coinText;
 [SerializeField]
 globals coinScript;
 [SerializeField]
 public float speed;
 public int countdown;

 void Awake()
 {
     countdown = globals.allCoinsCollected;
     if (coinText != null && coinScript != null)
     {
         coinText.text = countdown.ToString();

     }
 }
 IEnumerator MyCoroutine()
 {
     
     while (countdown >= 1)
     {
         yield return new WaitForSeconds(1.0f);
         countdown--;
     }
 }
   
 void Update()
 {
     StartCoroutine("MyCoroutine");
     coinText.text = countdown.ToString();
 }

}

and the text and global script are set in the editor, as earlier mentioned. It displays the correct amount of coins collected but then counts down to -114