if statement.... condition is a string but if true i want to preform a calculation

I would like to make a if statement my condition is that if the it said yes in column A then add 5 to the value on column b if no then subtract 5

A      b   
yes   10   
no    13

so for example my result would appear in column called C like this

A      b   c
yes   10   15
no    13   8

please help :)