<p id="sum">0</p>
<div id="items">
  <p><input type="checkbox" class="item item-1" value="5" />5</p>
  <p><input type="checkbox" class="item item-2" value="14" />14</p>
  <p><input type="checkbox" class="item item-3" value="3" />3</p>
  <p><input type="checkbox" class="item item-4" value="8" />8</p>
</div>

items = Array.prototype.slice.call(document.querySelectorAll('.item'))
function add() {
  let sum = 0
  items.forEach(item => {
    if(item.checked) {
      sum += parseInt(item.value);
    }
  })
  document.querySelector('#sum').innerHTML = sum;
}
items.forEach((item) => {
  item.addEventListener('click', add);
});