String to number conversion using javascript

this is my html code but the problem is that if i add two strings it always gives the 50 as the addition.

 function add()
  {
   var fname=document.getElementById("fname");
   var lname = document.getElementById("lname");

   var fstring = fname.toString();
   var lstring = lname.toString();

    var fl = fstring.length;
    var ll = lstring.length;  

   var f1 = parseInt(fl,10);
   var l1 = parseInt(ll,10);
  document.getElementById("result").innerHTML = f1 + l1;
   }
</script>