PHP 원의 넓이 구하기

<form method="post" action="cycle.php">
<input type="text" name="number">
<input type="submit" value="send">
<input type="reset" value="clear">
</form>

cycle.php

<?php
$sum=$_POST["number"] * $_POST["number"] * M_PI;
printf("%d", $sum);
?>