How to bind total in footer of gridview

       For n = 0 To GridView2.Rows.Count - 1
                totalpayment += Val(GridView2.Rows(n).Cells(4).Text)
                totalred += Val(GridView2.Rows(n).Cells(9).Text)
            Next
            'Dim totalpayment As Double = Convert.ToInt64(dt.Compute("sum(PaymentAmt)", ""))
            If GridView2.Rows.Count > 0 Then
                GridView2.FooterRow.Cells(3).Text = "<b>Grand Total</b>"
                GridView2.FooterRow.Cells(4).Text = Format(totalpayment, "<b>0.00</b>")
                GridView2.FooterRow.Cells(8).Text = "<b>Grand Total</b>"
                GridView2.FooterRow.Cells(9).Text = Format(totalred, "<b>0.00</b>")
            End If
        End If
        If Request.QueryString("ete") = "1" Then
            Response.ContentType = "application/ms-excel"
            Response.AddHeader("Content-Disposition", "inline;filename=" & rptName & ".xls")
        End If