challenge ujikom

<form method="post">
    <table>
        <tr>
            <td>Nilai UTS</td>
            <td><input type="number" min="0" max="100" name="uts"></td>
        </tr>
        <tr>
            <td>Nilai UAS</td>
            <td><input type="number" min="0" max="100" name="uas"></td>
        </tr>
        <tr>
            <td>
                <input type="submit" value="Hitung">
            </td>
        </tr>
    </table>
</form>

<?php
if (isset($_POST['uts']) && isset($_POST['uas'])) {
    if ($_POST['uts'] == "" || $_POST['uas'] == "") {
        echo "Nilai tidak boleh kosong";
    } else {
        $uts = $_POST['uts'];
        $uas = $_POST['uas'];
        $nilai = ($uts * 0.4) + ($uas * 0.6);
        echo "Nilai Akhir : $nilai" . "<br>";
        if ($nilai >= 80) {
            echo "Grade : A";
        } elseif ($nilai >= 70) {
            echo "Grade : B";
        } elseif ($nilai >= 60) {
            echo "Grade : C";
        } elseif ($nilai >= 50) {
            echo "Grade : D";
        } else {
            echo "Tidak lulus";
        }
    }
}
?>