Android back button / Pause and exit game

Hello, Could some one help me out. I am a beginner with Unity and C# programming. Currently working on my own project. The game is portrait mode which consists of 3d Game objects like sphere and cube where the sphere is a rigid body with gravity enabled and the cube recurring from the bottom of the screen. 1) I would like to add a exit menu (exit game(yes/no) ) which is enabled on single click of back button in android and the game must be paused. a) Yes - go to main menu b) No - Resume game 2) on second click of back button the application must exit.

I have tried many ways, but could not succeed.

Thank you.