/* write a function tha sum the choice of checkbox controls */
function sumChecked() {
  var sum = 0;
  var checkboxes = document.getElementsByTagName('input');
  for (var i = 0; i < checkboxes.length; i++) {
    if (checkboxes[i].type == 'checkbox' && checkboxes[i].checked) {
      sum += parseInt(checkboxes[i].value);
    }
  }
  return sum;
}