How can I solve this indexoutofrange exception plz help me

<asp:gridview id="Gridview1" runat="server" emptydatatext="Data is not available" onrowdatabound="Gridview1_RowDataBound" autogeneratecolumns="false">
    <columns>
        <asp:TemplateField HeaderText="ID">
            <itemtemplate>
                <asp:TextBox ID="txtID" runat="server" Text='<%#Eval("ID") %>' />
            
        
        <asp:TemplateField HeaderText="Quantity">
            <itemtemplate>
                <asp:TextBox ID="txtQuantity" runat="server" Text='<%#Eval("Quantity","0") %>' />
            
        
        <asp:TemplateField HeaderText="Rate">
            <itemtemplate>
                <asp:TextBox ID="txtRate" runat="server" Text='<%#Eval("Rate","0") %>' />
            
        
        <asp:TemplateField HeaderText="Total">
            <itemtemplate>
                <asp:TextBox ID="txtTotal" runat="server" Text='<%#Eval("Total","0") %>' />

protected void Gridview1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        int index=0;

        //if (e.Row.RowType >= null)
        //{
            TextBox txtRate = (TextBox)Gridview1.Rows[index].Cells[1].FindControl("txtRate");
            TextBox txtQuantity = (TextBox)Gridview1.Rows[index].Cells[2].FindControl("txtQuantity");
            TextBox txtTotal = (TextBox)Gridview1.Rows[index].Cells[3].FindControl("Total");
            int R = int.Parse(txtRate.Text);
            int Q = int.Parse(txtQuantity.Text);
            int T = int.Parse(txtTotal.Text);

            T = Convert.ToInt32(R * Q);
            txtTotal.Text = T.ToString();
        //}
    }
}