In your Manifest -:

<meta-data android:name="com.google.android.gms.version"
            android:value="@integer/google_play_services_version" />

In your gradle -:

implementation 'com.google.android.gms:play-services-ads:23.4.0'

For latest version -: https://mvnrepository.com/artifact/com.google.android.gms/play-services-ads

Enjoy!!