Variables for Buttons | Next button to enable

Hello All,

Slide1 - 4 buttons (A,B,C,D), Clicking each button will take me to slides (Slide-A, Slide-B, Slide-C, Slide-D). Each slide will have a Back button, Prev and Next button will be disabled. On clicking the back button it will return to Slide1. Next button in the Slide1 should be enabled only after the 4 slides are visited. Please help me how to do this. Thanks

B. Arun Moses