sum to only return +ve numbers

I am trying to figure out how to get a simple sum A1-A2 to only return +ve numbers if the outcom is -ve then I need it to return zero.

Any help appreciated

Thanks i have assumed you meant a1-a2 and not a1+a2.

I dont suppose you know how to expand this so if a2 = 0 then it returns zero as well do you?