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: Wrong Answer)

while n = gets.to_i && n
  def fact(n) n > 0 ? n * fact(n - 1) : 1 end
  puts fact(n)
end