Instead of showing interstitial ad on back press you can set it when screen initializes.
For example a user had pressed back button when he was in Screen 2 and Screen 1 was opened. As you have put ad on the Screen 1 initialize, ad will be shown.
Ad Implementation
If you want that ad not to be shown when the Screen 1 of your app is opened for first time then set start value when screen 2 is back pressed and make block such that if start value equals to some value then show ad.
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