Unity3D显示添加每5次重启

我试图让它在你重新启动游戏时显示5次广告。这是我目前得到的。这行不通。

    private int restartNum {
        get {
            return PlayerPrefs.GetInt ("restartNum");
        }
        set {
            PlayerPrefs.SetInt ("restartNum", value);
        }
    }
    public void OnButtonClick(string sceneName)
    {
        restartNum += 1;
        if (restartNum == 5) {// adding this makes it not work
            ShowAd ();// This works by itself
            restartNum = 0;
        }
        SceneManager.LoadScene (sceneName);
    }