Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <link type="text/css" rel="stylesheet" href="stylesheet.css"/>
    <title>Simple Calculator</title>
</head>
<body>
 <script type="text/javascript">
     function Go() {
         var first = +(document.getElementById('FirstNumber').value);
         var second = +(document.getElementById('SecondNumber').value);
         var action = document.getElementById('Action').value;
         var res;
         alert(res);
         alert(action);
          switch (action) {
              case 'sum' :
                  res == +first + +second;
                  break;
              case 'minus' :
                  res == +first - +second;
                  break;
              case 'multi' :
                  res == +first * +second;
                  break;
              case 'div' :
                  res == +first / +second;
                  break;
              default :
                  alert('error');
          }
         alert(res);
         alert(action);


     }
 </script>
 <input type="text"  value="0" name="first">
 <input type="text"  value="0" name="second">
 <p>
     <input type="radio"  value="sum" name="action"> Сумма
     <input type="radio"  value="minus" name="action"> Вычитание
     <input type="radio"  value="multi" name="action"> Умножение
     <input type="radio"  value="div" name="action"> Деление
 </p>
 <input type="button" value="Go" id="Go" onclick="Go()" >
</body>
</html>