you need to do that server side once the form has posted.
<?php
if($_SERVER['REQUEST_METHOD'] == "POST") {
$var1 = (isset($_POST['element1']))?intval($_POST['element1']):0;
$var2 = (isset($_POST['element2']))?intval($_POST['element2']):0;
$theSum = ($var1 + $var2);
$_POST['totalElement'] = $theSum;
}
?>