How to get the last two characters in string

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

    CodeBehind="Default.aspx.cs" Inherits="fetchlastcharacter._Default" %>



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

</asp:Content>

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

    <h2>

        Welcome to ASP.NET!

    </h2>

    <p>

        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

    </p>

    <p>

        &nbsp;</p>

    <p>

        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>

    </p>

    <p>

        <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />

    </p>

    </asp:Content>

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;



namespace fetchlastcharacter

{

    public partial class _Default : System.Web.UI.Page

    {

        protected void Page_Load(object sender, EventArgs e)

        {



        }



        protected void Button1_Click(object sender, EventArgs e)

        {

           

            string retString = null;

            retString =TextBox1.Text.Substring(16, 45);

            TextBox2.Text = retString;

           

        }

    }

}