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
def factorial(n)
 if n != 1
  return n*factorial(n-1)
 else
  return 1
 end
end

while gets
 puts factorial($_.to_i)
end