This calculator uses a modified version of one of the Mindstorms tutorial's code to add how many times you press one touch sensor to how many times you press another touch sensor. To subtract, you have to add negative numbers.

Oh and sorry for the poor picture quality, I took everything on an iPhone camera :P

Step 1: Parts

Parts:
Two touch sensors
8 black pegs
2 "L" bars
Mindstorms NXT brick
2 connector cords for sensors

Step 2: Add the L Bars

Attach the L bars to the NXT with the little peg things.

Step 3: Add the Sensors

Attach the sensors to the L bars with the leftover peg things.

Step 4: Plug the Sensors Into the NXT

Plug the left sensor into port 1 and the right sensor into port 2. I put two longer pegs in the NXT to keep the wires in place, but it won't affect anything if you leave them out.