这个JavaScript怎么敲嗷熬奥啊

img


<!DOCTYPE html>
<html>
<head>
    <title>判断成绩等级</title>
</head>
<body>
    <script>
        var yw_num = prompt("请输入语文成绩");
        var ss_num = prompt("请输入数学成绩");
        var yy_num = prompt("请输入英语成绩");
        var hx_num = prompt("请输入化学成绩");
        var ls_num = prompt("请输入历史成绩");
        var num = (parseInt(yw_num) + parseInt(ss_num) + parseInt(yy_num) + parseInt(hx_num) + parseInt(ls_num)) / 5;
        switch (num) {
            case num >= 90:
                alert("优秀");
                break;
            case num >= 80:
                alert("优良");
                break;
            case num >= 70:
                alert("良好");
                break;
            case num >= 60:
                alert("合格");
                break;
            default:
                alert("不合格");
                break;
        }

    </script>
</body>
</html>