function sumValues()
{
  var num1, num2, res;
  num1 = document.getElementById("txtnum1");
  num2 = document.getElementById("txtnum2");
  res = document.getElementById("txtres");
  res=num1+num2;
  document.getElementById("txtres).innerHTML = res;
}

</script>


<form name="formcalc">
<input type="text" id="txtnum1"><br>
<input type="text" id="txtnum2"><br>

<input type="button" value="Calculate" onClick="sumValues()"><br>
<input type="text" id="txtres"><br>
</form>