javascript textbox_changed value

这是我的ASPX代码

<%@ Page Title="About Us" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" CodeFile="About.aspx.cs" Inherits="About" %>
<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
<script language="javascript" type="text/javascript">
        function sum(t1)
        {
            var txt1 = document.getElementById('MainContent_TextBox2').value;
            var result = parseInt(txt1) / 2;
            var result1 = parseInt(result);
            if (!isNaN(result))
            {
                document.getElementById(t1).value = result1;
            }
        }
</script>
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
<asp:TextBox ID="TextBox2" runat="server" onkeyup="sum('MainContent_TextBox3')"</asp:TextBox>
<asp:TextBox ID="TextBox3" runat="server" ReadOnly="True"></asp:TextBox>
<br />
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click"/>
<asp:TextBox ID="TextBox4" runat="server"></asp:TextBox>

我的c#代码是

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class About : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        TextBox4.Text = TextBox3.Text;
    }
}

问题是，当我点击按钮1时，没有错误，TextBox4为空，我希望当我点击选项1时，textbox3的值在TextBox4中传输。