You can get the values with use of ID. But ID should be Unique.
<body>
<h1>Adding 'a' and 'b'</h1>
<form>
a: <input type="number" name="a" id="a"><br>
b: <input type="number" name="b" id="b"><br>
<button onclick="add()">Add</button>
</form>
<script>
function add() {
a = $('#a').val();
b = $('#b').val();
var sum = a + b;
alert(sum);
}
</script>
</body>