Trying to get ads to play after 3 game overs

Having all sorts of problems trying to get ads to play after 3 game overs, could anyone please help.
static int i = 0;

// Use this for initialization
void Start()
{
    if (GameManager.singleton.isGameOver == true && i % 4 == 0) 
    {
        ShowAd ();      
    }
    i++;
}

public void ShowAd()
    {
        if (Advertisement.IsReady())
        {
            Advertisement.Show();
        }
    }

(this is the closest thing I got to working, but ad also plays when game play is started.

private int i = 0;

// Use this for initialization
void Start()
{
   
    if (i % 4 == 0)
    {
  
        ShowAd();
      
    }
    i++;
}
// Update is called once per frame
void Update ()
{
   
    if (GameManager.singleton.isGameOver == true) 
    {
        if (i % 4 == 0)
        {
            
            ShowAd();
            i++;
        }
    }