JavaScript function to sum (add) TextBox values

I am developing a function which sum two textbox value which have a problem. Problem Is that  it together textbox value instead of doing sum e.g if I enter first textbox3 and second textbox 4 now in the third textbox it give answer 7 but it give the 34 means it combines the value of text box please correct my function following my function and textboxes.

///////function////////////////  
<script type="text/javascript">
        function Getvalue() {   
          var first = ($('#TextBox1').val());
          var next = ($('#TextBox2').val());
          var result = first + next;
         result = ($('#TextBox3').val(result));
          }; 
    </script>
 ///////////////////// textboxes and button//////////////////
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
        <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
   <input id="Button4" type="button" value="+" onclick="return Getvalue();"/>

thanks in advanced.