JavaScript - Aspnet

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script language="javascript" type="text/javascript">
        function sum() {
            var x = document.getElementById('<%= txtA.ClientId %>');
            var a = x.value;
            var y = document.getElementById('<%= txtB.ClientId %>');
            var b = y.value;
            var c = a + b;
            alert(c);
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:TextBox ID="txtA" runat="server" />
        <asp:TextBox ID="txtB" runat="server" />
        <span id="a" />
        <input type="button" onclick="javascript: return sum();"  />
    </div>
    </form>
</body>
</html>