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 PHP (judge verdict: Accepted)

<?php
(int)$inLine = trim(fgets(STDIN));
$ans = 1;
for ($i = $inLine; 0 < $i; $i--) {
     $ans = $ans * $i;
}
echo $ans.PHP_EOL;
?>