I get a little frustrated, because the web is AMAZING for this. Type this in anywhere. Anywhere!

  <input id=foo></input>
  <input id=bar></input>
  <button onclick="sum()">sum</button>
  <div id=result></div>
  <script>
    function sum() {
      result.innerText = parseFloat(foo.value) + parseFloat(bar.value);
    }
  </script>

That's full of "bad practices" but for the use case you're talking about, who cares?