Calculates the Greatest Common Divisor.

        :param a: the first number.
        :param b: the second number.
        :return: GCD(a, b)

def gcd(a, b):
        """
        Calculates the Greatest Common Divisor.

        :param a: the first number.
        :param b: the second number.
        :return: GCD(a, b)
        """
        while b:
            a, b = b, a % b

        return a