How to pass a variable as an integer to a function?

How to pass a variable as an integer to a function?

Let's say I have this function:
  1. function add(x, y) {
  2.   var add = x + y;
  3.   return add;
  4. }
And I have this code:

  1. $(select).change(function(){
  2.   // Get select values
  3.   var select_first = $("select#first").val();
  4.   var select_second = $("select#second").val();
  5.   // Add select values
  6.   var add = add(select_first, select_second);
  7.   $("p#result").html(add);
  8. });
So let's say the value of select_first is "1", and value of select_second is also "1". But when they are passed to the add function, the returned value is 11 and not 2. So what I understood is that it took the values as strings instead of integers. What should I do so that select_first and select_second are integers instead of string?