def factorial(n):
    if n == 0 or n == 1:
        return 1
    else:
        result = 1
        for i in range(2, n + 1):
            result *= i
        return result
while True:
    try:
        num = int(input("Enter an integer: "))
        if num < 0:
            print("Factorial is not defined for negative numbers.")
        else:
            print(f"The factorial of {num} is: {factorial(num)}")
        break
    except ValueError:
        print("Please enter an integer.")