2s Complement Overflow Rule

• ONLY if both number are the same magnitude (ie. they both have 1s or both have 0s in the most significant digit), AND the most significant digit (the leftmost digit of the answer, not the carry out!) of the answer is different, is the flag set to 1 (some error, overflow or underflow)

• so how do we build a circuit that we can add to a Full Adder Stack to solve this problem