def gcd(a: int, b: int) -> int:
    """
    gcd returns the greatest common divisor

    Args:
        a (int): first number
        b (int): second number

    Returns:
        int: great common denominator
    """
    if a == 0:
        return b
    if b == 0:
        return a
    return gcd(b, a % b)