<?php
function fact($n) {
	$sol = 1;
	for ($i=1; $i <= $n; $i++) {
		$sol *= $i;
	}
	return $sol;
}
$handle = fopen('php://stdin', 'r');
$n = (int)trim(fgets($handle));
echo fact($n);

// for n=25 I am getting 1.5511210043331E+25
?>