<script>

var txt1=parseInt(document.getElementById("txt1").value);

var txt2=parseInt(document.getElementById("txt2").value);

var se=document.getElementById("vu").value;

var d;

function result(){

switch(se){

case "+":

d=txt1 + txt2;

break;

case "-":

d=txt1 - txt2;

break;

case "*":

d=txt1 * txt2;

break;

case "/":

d=txt1 / txt2;

break;

}

document.getElementById("sum").value=d;

}

</script>

<input type="text" id="txt1">

<select id="vu">

  <option value="+">+</option>

  <option value="-">-</option>

  <option value="*">*</option>

  <option value="/">/</option>

</select>

<input type="text" id="txt2">

<input type="button" value="=" onclick="result()">

<input type="text" id="sum">