# File: nfact.py
# Program to compute n!
#
# Matt Bishop, MHI 289I, Winter 2018

#
# compute n!
# assumes n is a non-negative integer
#
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
#
# input number
try:
    n = int(raw_input("Factorial: "))
except:
    print "Need an integer"
else:
    # it better be non-negative!
    if n < 0:
        print "Need a non-negative integer"
    else:
        # compute n! and announce it
        print "%d! =" % n, fact(n)