Increase one cell by decreasing another.

Is it possible to have a cell increased by 1 and another decreasing by 1 in a simple cell formula.

Eg:

I have an inventory stock sheet that is very simple. I have a maximum stock column C, a stock on hand (available) column A and a stock sold column B.

Every time I sell an item and punch it into column B it should automatically decrease the amount in column A by the number that I punch into B.
I am not concerned about the maximum stock column but would like an easy formula just to work out the amount that I have available so I know when to order more stock.

So when I punch in a 5 on B2 it should decrease the value of A2 to 1. And when I purchase stock again and I punch in say a 5 on A2 it should decrease B2 to a 1.
Not sure if this is possible.

Any help will be much appreciated

****A* B *C
1 *2 *4 *6
2 *4 *2 *6
3 *8 *6 *14
4 *0 *6 *6

Thank you for the reply Miroslav.

Yes that is exactly how I would want it to work. It wouldn't be any problem to actually add more columns in. But I am not too sure how to make the formula then. My column C is just to give me an indication of the maximum Stock that I should have, that doesn't really have to be used.

Regards
Mark Harrison