It's mathematically correct, but integers use truncating (floor) division. So you can lose a fractional 0.5 from both a/2 and b/2, which would have added up to

It's mathematically correct, but integers use truncating (floor) division. So you can lose a fractional 0.5 from both a/2 and b/2, which would have added up to 1, and now your result is off by one from (a+b)/2. In general, even with floating point numbers you can get different results by rounding implicitly or intentionally the intermediate values versus the end result.