How to use google play service 31 to admob unity 4.6.9

Hi every one !

I'm trying to add google admob to my project. I'm using google admob plugin. But I can't find google play service lib with my google play service revision 31.

I'm finding solution and following Athalos solution. But I have error:

1. AndroidJavaException: java.lang.ClassNotFoundException: com.google.android.gms.ads.AdSize
2. at UnityEngine.AndroidJNISafe.CheckException()[0x00000]in<filename unknown>:0
3. at UnityEngine.AndroidJNISafe.CallStaticObjectMethod(IntPtr clazz,IntPtr methodID,UnityEngine.jvalue[] args)[0x00000]in<filename unknown>:0
4. at UnityEngine.AndroidJavaObject._CallStatic[AndroidJavaObject](System.String methodName,System.Object[] args)[0x00000]in<filename unknown>:0
5. at UnityEngine.AndroidJavaObject.CallStatic[AndroidJavaObject](System.String methodName,System.Object[] args)[0x00000]in<filename unknown>:0
6. at UnityEngine.AndroidJavaObject.FindClass(System.String name)[0x00000]in<filename unknown>:0
7. at UnityEngine.AndroidJavaClass._AndroidJavaClass(System.String className)[0x00000]in<filename unknown>:0
8. at UnityEngine.AndroidJavaClass..ctor (System.String className)[0x00000]in<filename unknown>:0
9. at GoogleMobileAds.Android.Utils.GetAdSizeJavaObject(GoogleMobileAds.Api.AdSize ad

I'm researching my error and google play service of my project can't be found.

So what can I do to add or find google play service lib with my google play service revision 31.

Pls help me