The program will require the following:
1) Determine pressure applied onto the screen. Whether it be an object, or the user pushing his/her finger onto the screen, the programmer must know how to determine how much pressure is being applied.
2) When an object is resting on the screen, the program must detect the area of the object as it rests on the screen.
3) Create a simple calculator.
I'm not quite sure if items 1 & 2 are possible, but if you are positive that you can do this, please contact me.