Cell absorb value that user input

Hello EVERYONE, i am working on a little "inventory project" and i have a question.
Is it possible to make a cell that will absorb the value that user input, so that the next user do not have to summon by previous number. (you will see on the picture)

Example: Element C-10 is already 28 pieces out. Now if i take 5 more, i have to sum/add up 28+5=33, and putt 33 in CELL-G5.

I want to make that when user input the number in one of the G-cells. It automatically substract and make the cell empty again.

What will be the easyest way to do that?

Thank you very much for your time!

Best Regards, Gasper