Performing subtraction between fields

[b]

Hello,

For simplicity's sake, let's say I have three fields, A, B and C, all
of which are integers. I'd like the value of C to be equal to A less B
(A-B). Is there a way I can perform this calculation? I'm guessing it
would happen when I INSERT a row and specify the values for A and B.
Feel free to direct me to the fine manual I should have read.

Thank you.