I've got this strange spreadsheet I need to do for work, I've been looking at this for hours and can't think of what I need to do to make it work...

this is what it needs to do.

The user needs to select the a number out of a drop down list, the next user that comes along needs to pick a diffrent number out of that same drop down list, (this process can happen up to 20 times), those 2 numbers need to be added together

I need this list to keep track out samples stock.

so original stock (20) - [sum of numbers entered in the drop down list] = left over stock

my question is simple how can I get those numbers to add them self together rather then just update.