> since the decimal average is -0.5

The C standard says: When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded (This is often called ‘‘truncation toward zero’’).

So it should be 0 (as per C standard, not sure what C++ standard says)

The question is midpoint, not division, so that's irrelevant in the first place, and even if were division, the standard is wrong.