Notcalc (Calculate the wrong answer)
RProgN, 4 3 1 Byte
Crossed out 4 is still 4 ;(
E

The most simple of solutions, compares if A and B are equal. Pushes true, which RProgN sees as 1, if they are the same, or false aka 0 otherwise.
Test Cases
0+0 = 1
1+0 = 0
0+1 = 0
1+1 = 1

Try it online!