Cumulative value in a single cell
 

Dear xls xperts,
I want the solution below.

cell A
Cell B
Cell C
Suppose, i put 50 in cell B, Cell A will take it and add the number
frequently. again if I delete or put another 50 on cell B, even then cell A
will show me 100(50+50) whereas Cell C will only minus the number that Cell
B helps Cell A to add. thanks

Shah Alam