inventory cells
 

I am using excel 2000 as an inventroy tool,my problem is finding a formula
that will allow me to take a cell (C) (quanity on hand) and through 2 other
cells (A),(B) either add and subtract from that number multiple times, which
show an increase or decrease as numbers are inputed. Then at the end of the
day clear cells (A) and (B) and have cell (C) show the correct inventory.

Any help would be greatly appreciated.