public class KillGhostAfterPowerUp : MonoBehaviour {
            public bool killCommand = false;
        public float startingTime = 0f;
            private float powerUpExpireTime = 2f;
            private float powerTimer = 0;
            void OnCollisionEnter2D (Collision2D other){
                    if (killCommand == true) {
                            if (other.gameObject.name == "Ghost") {
                                    Destroy (other.gameObject);
                            }else if(time.time - startingTime > powerUpExpireTime){
                                    killCommand = false;
                            }
                    }
            }
    }