Multiply a Number by 2

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>First Steps Into PHP</title>

</head>
<body>
<?php
    if (isset($_GET['num'])) {
        $n = intval($_GET['num']);
        $n = $n * 2;
    }
?>
<form>
    N: <input type="text" name="num" value="<?php echo$n?>"/>
    <input type="submit" />
</form>
</body>
</html>