Problem:
Factorial

Write a program which reads an integer n and prints the factorial of n. You can assume that n ≤ 20.

Input

An integer n (1 ≤ n ≤ 20) in a line.

Output

Print the factorial of n in a line.

Sample Input

5

Output for the Sample Input

120

Solution in Ruby (judge verdict: Accepted)

#!/usr/bin/ruby
# -*- coding: utf-8 -*-
=begin
=end
def fac(n)
  case n
  when 0
    1
  when 1
    1
  else
    n*fac(n-1)
  end
end

n = gets.to_i
puts fac(n)