I want to be able to add what ever number has been subtracted from one colum
to another colum.

Example: colum A colum B

210 25
200 35
190 45
250 45
230 65
200 95

When I change the number in colum A to a lower number. I want
it to automatically add to colum B. But if I add to colum A, I want nothing
to happen.
Colum A is our in stock amount. Colum B is total used or sold.
This is probably a simple thing to do, but so far I have not found the
formula, or macro to do it.

I Thank You for any Help you may tell me.

Thank You,

Craig Alan Johnson Sr.