return add on value

My jquery function below does not work for me. I want this to read my value for
   var s = 0; for example if I have
   var s = 10;
then I want to return 10 and then start adding once i enter a value.
so if I enter a value in the text box as 2 then it will add the 10 plus 2 in realtime to 12
I just do not want this to add all the values in the text box only adding with var s will be.
The reason for this I do not have one big long page with all the textboxes because I am using a pager.
I can get the total value of all the textboxes and pass it to Vars and then if I type a value it will add on to this.


$(document).ready(function() {
                      //add functions here
                       $('input[id^="txtquantity"]').live('keyup', function() {
                         if (String($(this).val()).length < 1) return false;
                         getSum();
                       });
                 });
          $(document).ready(function() {
              getSum();
          });

     function getSum() {
              var s = 0;
              $('input[id^="txtquantity"]').each(function() {
                  //alert($(this).val());
                 var v = $(this).val();
                 s = s + (isNaN(v) ? 0 : parseInt(v));
           });
              $('#lblSum').html(s);
          }