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)

#encoding:utf-8 No.0019 2014.10.2
def fact(n)
  if n != 1
    return n * fact(n-1)
  else
    return 1
  end
end

while gets
  puts fact($_.to_i)
end