Greatest Common Factors of Fractions

I need help finding GCF of this fraction program

Code:

#include <iostream.h>
#include <math.h>

int main()
{
	// Declare variables
	int numerator;		// Numerator for fraction
	int denominator;	// Denominator for fraction
	int a, b, i;		// a = denominator, b = numerator. i = GCF
	int ii;				// GCF
	int hold;			// holds output screen
	
	// Get fractions from user
	cout << "Enter the numerator : ";
	cin  >> numerator;
	cout << "Enter the denominator : ";
	cin  >> denominator;
	
	// Invalid fractions
	if (denominator == 0)
	{
		cout << "Invalid Operation. Cannot divide fraction by 0." << endl;
	}
	
	if (numerator == 0)
	{
		cout << "Invalid Operation. Numerator cannot be zero." << endl;
	}
	
	// Reduce Fractions with For loop
	a = denominator;
	b = numerator;
	for (i = a * b; i > 1; i--)
        {
                if ((a % i == 0) && (b % i == 0))
                {
                        a /= i;
                        b /= i;
                }
        }
    cout << endl;
 	
 	
 	
    // Print results to screen
    cout << "Numerator   = " << b << endl;
    cout << "---------     - " << endl;
    cout << "Denominator = " << a << endl;
    cout << endl;
    //cout << "Greatest Common Factor : " << i << endl;
    cin  >> hold;
    return 0;
}

Euclid's algorithm of finding GCD