need help with C++

The Greatest Common Divisor (gcd) of two or more integers, when at least one of them is not zero, is the largest positive integer that divides the numbers without a remainder. for instance, the GCD of 8 and 12 is 4. Write a function named computeGCD that takes two positive integer arguments and returns the greatest common divisor of those two integers. If the function is passed an argument that is not greater than zero, then the function should return the value -1 to indicate than an error occurred