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)

def kaijo(n)
  if n <= 1
    return 1
  else
    return n * kaijo(n - 1)
  end
end
puts kaijo(gets.to_i)