Comes Before

 This equation lets you choose a number and then see what follows that number. We're using counting integers here:

10, 9, 8, 7, 6, 5,4, 3, 2, 1, ...

For example:   Enter the number 5. 

What comes next?  The number 4 will be displayed.

This will work with negative numbers too, if you're a math wizard!