function random(max) {
    return Math.floor(Math.random() * Math.floor(max));
}
function start() {
    var form = document.getElementById("name_form");
    var name = form.elements[0].value;
    var score = 0;
    alert("Let's begin!");
    for (let i = 0; i < 4; i++) {
        var num1 = random(10);
        var num2 = random(10);
       var operator = random(2);
       if (operator == 0) {
           var question = prompt(num1 + " + " + num2 + " =");
           var answer = num1 + num2;
       }
       else {
           var question = prompt(num1 + " - " + num2 = " =");
           var answer = num1 - num2;
       }
       if (question == answer) {
           alert("Correct!");
           score++;
       }
       else {
           alert("Incorrect ):");
       }
    }
    document.write(score);
}