<input id="input1" onkeyup="fn1()" />
  <input id="input2" />
  <input id="input3" />
  <select id="select">
    <option value="+">+</option>
    <option value="-">-</option>
    <option value="/">/</option>
    <option value="*">*</option>
  </select>
  <input type="button" onclick="sum()" value="dfsdf">

<script>
function fn1() {
  var input1 = document.getElementById("input1");
  var input2 = document.getElementById("input2");

  input2.value = input1.value
}

function sum() {
var input1 = document.getElementById("input1");
  var input2 = document.getElementById("input2");
  var input3 = document.getElementById("input3");
  var select = document.getElementById("select");

  if(select.value == "+" )
  {
    input3.value = +input1.value + +input2.value
  }


  if(select.value == "*") {
    input3.value = +input1.value * +input2.value;
  }
 if(select.value == "/"){
   input3.value = +input1.value / +input2.value;
 }

  input3.value = calc()


  //alert(select.value)

}

function calc() {
  return Math.sqrt(Math.sqrt(5)) + Math.pow(10, 2);
}


</script>