Android : Implementing Google ads via Android-Studio and the 'New' Google Play Services SDK

Using Android Studio to compile Google Ads via Google Play Services SDK. When I add this tag to my Android manifest file:

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

I get a compiler error that the integer value for 'google_play_services_version' is not defined - the text inside these quotes is also error red. What is the problem?