<script>
  function calc()
  {
  const firstnumber = document.getElementById("first").value;
  const secondnumber = document.getElementById("second").value;
  const IsAdded = document.getElementById("added").checked;
  var result = document.getElementById("result");

  let num = parseInt (firstnumber) + parseInt (secondnumber) + (IsAdded ? 10 : 0);
  result.innerHTML = num;
     
  }
</script>
<form>
    Liczba 1: <input type="text" id="first"><br>
    +<br>
    Liczba 2: <input type="text" id="second"><br>
    <input type="checkbox" id="added"> dodaj do wyniku 10<br>
    <input type="button" value="OK" onclick="calc()"><br>
</form>
Wynik: <span id="result">0</span>