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)

class Fixnum
  def factorial
    case self
    when 1 then 1
    else self * (self - 1).factorial
    end
  end
end

puts gets.to_i.factorial