ขอถามเรื่อง การเพิ่มข้อมูลลงไปใน Gridview เเล้วให้หา Total ลงใน footer หน่อยครับ

 protected void Button1_Click(object sender, EventArgs e)
        {
            
            int pay = Convert.ToInt32(TextBox2.Text) * Convert.ToInt32(TextBox4.Text);
            DataTable dt = new DataTable();
            dt.Columns.Add("id");
            dt.Columns.Add("list");
            dt.Columns.Add("count");
            dt.Columns.Add("unit");
            dt.Columns.Add("pay");
            dt.Columns.Add("allpay");
            dt.Columns.Add("amount");
            DataRow dr = null;
            if (ViewState["table"] != null)
            {
                for (int i = 0; i < 1; i++)
                {
                    dt = (DataTable)ViewState["table"];
                    if (dt.Rows.Count > 0)
                    {
                        dr = dt.NewRow();
                        dr["id"] = GridView1.Rows.Count + 1;
                        dr["list"] = TextBox1.Text;
                        dr["count"] = TextBox2.Text;
                        dr["unit"] = TextBox3.Text;
                        dr["pay"] = TextBox4.Text;
                        dr["allpay"] = pay;
                        dt.Rows.Add(dr);
                        GridView1.DataSource = dt;
                        GridView1.DataBind();
                    }
                }
            }
            else
            {
                dr = dt.NewRow();
                dr["id"] = GridView1.Rows.Count + 1;
                dr["list"] = TextBox1.Text;
                dr["count"] = TextBox2.Text;
                dr["unit"] = TextBox3.Text;
                dr["pay"] = TextBox4.Text;
                dr["allpay"] = Convert.ToInt32(TextBox2.Text) * Convert.ToInt32(TextBox4.Text);
                dt.Rows.Add(dr);
                GridView1.DataSource = dt;
                GridView1.DataBind();
            }
            ViewState["table"] = dt;
        }