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)

while line = gets
  n = line.chomp.to_i
  answer = 1
  2.upto(n) do |i|
    answer *= i
  end

  puts answer
end