How to focus cursor on textbox after page load.

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C#

ASP.NET

C#4.0

I have 4 textbox where first 3 textbox take integer value and 4th textbox show there addition result. I make all textbox's autopost back property true because I need to add immediately when text change into any textbox. Problem is when I type one textbox and try to switch another then the cursor focus lost from the 2nd textbox. Please someone help to get cursor on the next textbox after postback. Here I my code.

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
        <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
        <asp:TextBox ID="TextBox4" runat="server"></asp:TextBox>
    </ContentTemplate>
</asp:UpdatePanel>