def factorial(x):                            # ex. 10
    if (x == 0):
        print '1';
    if (x < 0):
        print 'Negative integers not allowed'

    temp = x                                 # temp = 10
    x = x-1                                  # x = 10-1 = 9
    subtotal = temp*x                        # 10*9 = 90
    total = subtotal

    while (x > 2):                                    # then:
        temp = x-1                           #  = 8          #  = 6             #  = 4           #  = 2
        x = x-2                              #  = 7          #  = 5             #  = 3           #  = 1
        subtotal = temp*x                    #  = 8*7        #  = 6*5           #  = 4*3         #  = 2*1
        total = total * subtotal             #  = 10*9*8*7   #  = 10*9*8*7*6*5  #  = 10*...*4*3  #  = 10*...*2*1

    print "The factorial of ",x," is ",total

number = input ("Enter a number: ")
factorial(number)