how to calculate value of two textfield and display its sum in third textfield autmotatically

<script>

function sum() {

      var txtFirstNumberValue = document.getElementById('txt1').value;

      var txtSecondNumberValue = document.getElementById('txt2').value;

      var result = parseInt(txtFirstNumberValue) + parseInt(txtSecondNumberValue);

      if (!isNaN(result)) {

         document.getElementById('txt3').value = result;

      }

}

</script>

<input type="text" id="txt1"  onkeyup="sum();" />

<input type="text" id="txt2"  onkeyup="sum();" />

<input type="text" id="txt3" />