App with activation interface

Question

Good day

 

I developed a WP8 app that requires the user to use an activation PIN before accessing the app. whenever the user input the right PIN they will have access to the app and it works fine

 

But the issue am facing is whenever the user closes the app and wants to reuse the app it asks for the PIN, i want the user to activate the app once with a correct PIN. The app not having to asks for the PIN when next they want to use the app unless they uninstall the app

 

How can i achieve this in c# ?

 

Regards and reply soon