Sum Only to Zero

Hello,

I am trying to figure out how to only auto sum up to zero. for instance, if F79+F80 = -100 then F81 would equal -100, however, if F79+F80 = 100 then F81 would equal 0.

I can figure out how to add only positive or negative numbers but what I need is to sum all numbers, positive nad negative. but only show the sum if it less than or equal to 0.

Thanks for any insight!