var sum = 0;
    for (let a = 0; a <= 10; a += 2) {
        sum += a;

    }
    document.write(sum);
    document.write("<br>")
    var b = Number(prompt("請輸入數(shù)字"));
    switch (b) {
        case 1:
        case 2: document.write("不及格");
            break;
        case 3: document.write("及格");
            break;
        case 4: document.write("良");
            break;
        case 5: document.write("優(yōu)秀");
            break;
        default: document.write("輸入數(shù)字有錯誤");

    }