how to set click counter in admob interstitial ad - counter

Question

I use the below code to show interstitial ad when clicking on a navigation menu item in my android app.But if I implement this code ad comes every time we click on the menu item.This will frustrate users.So how can I set a counter for this so that ad should come at second click on menu items and then for every 5 clicks etc.Any help would be most appreciated.
#Override
public void onDrawerItemSelected(View view, int position) {
// INTERSTITIAL AD IMPLEMENTATION
final InterstitialAd interstitialAd = new InterstitialAd(this);
interstitialAd.setAdUnitId(getString(R.string.ADMOB_INTERSTITIAL_UNIT_ID));
AdRequest requestForInterstitial = new AdRequest.Builder().build();
interstitialAd.loadAd(requestForInterstitial);
interstitialAd.setAdListener(new AdListener() {
#Override
public void onAdLoaded() {
Log.i("log-", "INTERSTITIAL is loaded!"); if (interstitialAd.isLoaded()) {
interstitialAd.show(); }
}
#Override
public void onAdClosed() {
Log.i("log-", "INTERSTITIAL is closed!");
}
#Override
public void onAdFailedToLoad(int errorCode) {
Log .i("log-", "INTERSTITIAL failed to load! error code: " + errorCode);
}
#Override
public void onAdLeftApplication() {
Log.i("log-", "INTERSTITIAL left application!");
}
#Override
public void onAdOpened() {
Log.i("log-", "INTERSTITIAL is opened!");
}
});

Answer 1

I solved it by following code:
if (counter == 4) {
Log.i("log-", "INTERSTITIAL is loaded!");
if (interstitialAd.isLoaded()) {
interstitialAd.show();
}
counter = 1;
}
else {
counter++;
}