How to Keep value of sum after deleting integer

I need help with what is probably a fairly easy VBA script, but i am new to VBA so, for lack of better words, I am stuck.

What i need is a script that will add two boxes together to get a sum in a third box, but where it is tricky for me is that i need the third box to retain the summed value even when either of the first two box values are deleted.

For example, say i have box A1=10, B1=40, and C1= SUM of A1 & B1 (so C1=50). Easy enough, but where im lost is that i want to be able to delete the value of either A1 or B1 and then have C1 still = 50, so then i can input another value into either A1 or B1 to continue to add to C1.

I hope this I explained this halfway decent, and any help would be greatly appreciated!