class InvalidFactorialArgumentException(Exception):
    def __init__(self, message):
        super().__init__()
        self.message = message

    def __str__(self):
        return self.message


def factorial(n):
    if not isinstance(n, int):
        raise InvalidFactorialArgumentException("Argument must be int")

    if n < 0:
        raise InvalidFactorialArgumentException("Argument must be >= 0")

    f = 1
    for i in range(n):
        f *= n
        n -= 1

    return f


try:
    print("Factorial of 4 is:", factorial(4))
    print("Factorial of 12 is:", factorial("12"))

except InvalidFactorialArgumentException as e:
    print("Error:", e)