# File: nfact.py
# Program to compute n!
#
# Matt Bishop, ECS 10, Fall 2012

#
# compute n!
#
def fact(n):
    # base case: 0! = 1 (by definition)
    if n == 0:
        return 1
    # recursion: n! = n * (n-1)!
    return n * fact(n-1)

#
# the main routine
#
def main():
    # input number
    try:
        n = int(input("Factorial: "))
    except:
        print("Need an integer")
        return

    # it better be non-negative!
    if n < 0:
        print("Need a non-negative integer")
        return

    # compute n! and announce it
    print("%d! =" % n, fact(n))
		
main()