Unity Adsのアセットストアパッケージを使っている際のコード（Start時にGame IDを使った初期化がはいる）

ShowUnityAdsWithAssetstorePackage.cs

using UnityEngine;
using UnityEngine.Advertisements;

public class ShowUnityAdsWithAssetstorePackage : MonoBehaviour
{
	public string GameId_iOS = "INPUT YOUR GAMEID FOR IOS";
	public string GameId_Android = "INPUT YOUR GAMEID FOR ANDROID";
	public bool TestMode = false;
	
	private string _gameId;
	
	void Start ()
	{
		if (!Advertisement.isInitialized)
		{
			#if UNITY_IOS
			_gameId = GameId_iOS;
			#elif UNITY_ANDROID
			_gameId = GameId_Android;
			#endif
			Advertisement.Initialize(_gameId, TestMode);
		}
	}
	
	public void ShowAd()
	{
		if (Advertisement.IsReady())
		{
			Advertisement.Show();
		}
	}
}