Adding numbers up

I use onChange in drop down menu, but when i wanna add all the number up with totalCost var in javascript i get answer undefined20 or other number and it doesnt add up.

<p id="test"></p>
   <p id="test2"></p>
   <select id="first" onChange="myFunction()">
   <option></option>
   <option value="20">Test1</option>
   <option value='100'>Test2</option>
   <option value='200'>Test3</option>

   </select>

    <script>
document.getElementById("test").onchange = function() {myFunction()};


function myFunction(){
     var x = document.getElementById("first").value;
     var totalCost;
     totalCost += x;
    document.getElementById("test").innerHTML = x;
    document.getElementById("test2").innerHTML = totalCost;
    }

</script>