# File: nfact.py
# Program to compute n!
#
# Matt Bishop, MHI 289I Fall 2022

#
# 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(input("Factorial: "))
except EOFError:
    pass
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("{0:d}! =".format(n), fact(n))