I have a list of items in my app. I want to give the 10 of them free with full functionality to show what type of service I’m offering. However, I don’t want to be the one who decides which item to show; I want the user to unlock which 10 he/she wants to open, instead. Is there any way to track the user so that when he deletes the app and installs it again, he’ll not be able to choose another (different) 10? I have no mandatory login screen to open the app.