How to integrate interstitial ad in 2nd tab?

Question

I'm creating an app having 2 tabs and Ad Mob, my question is how can I integrate interstitial ad in 2nd tab ? , means as per google AdMob policy Placing an interstitial ad after every user action is restricted, though I want to add fullscreen Ad when I swipe from 1st Tab to 2nd Tab and restrict the Ad when I swipe from the 2nd tab to 1st tab. How can I do that? Guide me with examples and corrections! Below is my current activity.

boolean firstResume = true;

 private ViewPager viewPager;
 private FBTabsAdapter mAdapter;
 private ActionBar actionBar;
 Handler handler1 = new Handler();
 private Context context;
 private int i = 0;
 private boolean exitAds = false;
 // private Fragment _fragment;

 // Tab titles
 private String[] tabs = { "TAB 1", " TAB 2" };

 @Override
 protected void onResume() {
  super.onResume();
  if (!firstResume) {

  } else {
   firstResume = false;
  }
 }

 @Override
 public void onTabReselected(Tab tab, FragmentTransaction ft) {
 }

 @Override
 public void onTabSelected(Tab tab, FragmentTransaction ft) {
  // on tab selected
  // show respected fragment view
  viewPager.setCurrentItem(tab.getPosition());
  if (tab.getPosition() == 1) {
   Log.e("UPDATED", "DOWNLOADS SELECTED");
   final DownloadsFragment fragment = ((DownloadsFragment) getSupportFragmentManager()
     .findFragmentByTag("android:switcher:" + R.id.pager + ":1"));
   fragment.updateDownloads();

   // if (i == 0) {
   // i++;
   // } else if (i > 0) {
   // }

  } // else
   // displayInterstitial();
 }

 @Override
 public void onTabUnselected(Tab tab, FragmentTransaction ft) {
 }

 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
  // Inflate the menu; this adds items to the action bar if it is present.
  getMenuInflater().inflate(R.menu.main, menu);
  return true;
 }

TABS ADAPTER

package xxxxxx.xxxxxxxxxxx.xxxxxxxxx.com;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;


public class ABTabsAdapter extends FragmentPagerAdapter {
    public ABTabsAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int index) {

        switch (index) {
            case 0:
                // Top Rated fragment activity
                return new TAB1Fragment();
            case 1:
                // fragment activity
                return new TAB2Fragment();
        }

        return null;
    }

    @Override
    public int getCount() {
        // get item count - equal to number of tabs
        return 2;
    }
    
}