The code is unarguably wrong.
average(INT_MAX,INTMAX) should return INT_MAX, but it will get that wrong and return -1.
average(0,-2) should not return a special error-code value, but this code will do just that, making -1 an ambiguous output value.
Even its comment is wrong. We can see from the signature of the function that there can be no value that indicates an error, as every possible value of int may be a legitimate output value.
It's possible to implement this function in a portable and standard way though, along the lines of [0].
[0] https://stackoverflow.com/a/61711253/ (Disclosure: this is my code.)
