I have A as constant. I subtract B from A giving me C. I want to be able to input different values(numbers) into B that accumulate. So 1st input 10 giving C new value. 2nd Input of 5 in B and having B become 15 which than updates C.

So A's value 100
Subtract B's value from A. Example 100-10
Giving C's value 100-10+90
Than input new value into B: 5 which adds to B original 10 making it 15
C's value should than become 85.

I want to be able to input new value in B as many times as I want.