How to get selected value from asp.net RadioButtonList using JavaScript?

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"

    CodeFile="Default.aspx.cs" Inherits="_Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">

    <script language="javascript" type="text/javascript">

        function GetSelectedValue() {

            var list = document.getElementById("<% =rdbtnLst.ClientID %>"); //Cleint ID of RadioButtonList

            var rdbtnLstValues = list.getElementsByTagName("input");

            var Checkdvalue;

            for (var i = 0; i < rdbtnLstValues.length; i++) {

                if (rdbtnLstValues.checked) {

                    Checkdvalue = rdbtnLstValues;

                    break;

                }

            }

            if (Checkdvalue) {

                alert(Checkdvalue.value);

            }

        }

    </script>

</asp:Content>

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">

    <div>

        <asp:RadioButtonList ID="rdbtnLst" AutoPostBack="true" runat="server" RepeatDirection="Horizontal">

            <asp:ListItem Selected="True">One</asp:ListItem>

            <asp:ListItem>Two</asp:ListItem>

            <asp:ListItem>Three</asp:ListItem>

            <asp:ListItem>Four</asp:ListItem>

        </asp:RadioButtonList>

        <asp:Button ID="Button1" runat="server" OnClientClick="GetSelectedValue()" Text="Button" />

    </div>

</asp:Content>