I am fairly new to using formulas but want a formula to do the following:

A B C
1 10 5 6
2 9 9
3

Figures entered into column A & B & Formula in column C.
Formula for C1 is A1 + B1 - A2
Help for the below
The formula for C2 is A2 + B2 - C3 but if C2 = A2 I want the value to be 0

Please help