C# DataGridView - Export to Excel อยากทราบวิธี SUM ข้อมูลที่ด้านล่างสุดของตารางครับ

private void Excel4_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            if (dataGridView4.DataSource == null)
            {
                MessageBox.Show("ไม่มีข้อมูลที่จะแสดง");
                return;
            }

            Excel.Application excel;
            Excel.Workbook wbook;
            Excel.Worksheet wsheet;
            excel = new Excel.Application();
            wbook = excel.Workbooks.Add();
            wsheet = wbook.ActiveSheet;

            excel.Visible = true;

            string title = "ประวัติการขายสินค้าของ  " + txthname.Text + "   " + txtfname.Text + "   " + txtlname.Text;
            wsheet.Range["A1", "H1"].Merge();
            wsheet.Range["A1", "H1"].Value = title;
            wsheet.Range["A1", "H1"].Font.Bold = 1;
            wsheet.Range["A3", "H3"].Font.Bold = 1;
            wsheet.Range["A1", "H1"].HorizontalAlignment = -4108;
            wsheet.Range["A3","A9999"].HorizontalAlignment = -4108;
            wsheet.Range["B3","B9999"].HorizontalAlignment = -4108;
            wsheet.Range["C3"].HorizontalAlignment = -4108;
            wsheet.Range["D3"].HorizontalAlignment = -4108;
            wsheet.Range["G3"].HorizontalAlignment = -4108;
            wsheet.Range["E3"].HorizontalAlignment = -4108;
            wsheet.Range["D3", "D9999"].HorizontalAlignment = -4108;
            wsheet.Range["F3", "F9999"].HorizontalAlignment = -4108;
            wsheet.Range["H3", "H9999"].HorizontalAlignment = -4108;

            wsheet.Range["B3", "B9999"].ColumnWidth = 10;
            wsheet.Range["C3", "C9999"].ColumnWidth = 13;
            wsheet.Range["D3", "D9999"].ColumnWidth = 15;
            wsheet.Range["E3", "E9999"].ColumnWidth = 14;
            wsheet.Range["F3", "F9999"].ColumnWidth = 18;
            wsheet.Range["G3", "G9999"].ColumnWidth = 15;
            wsheet.Range["H3", "H9999"].ColumnWidth = 18;

            wsheet.Range["C3", "C9999"].NumberFormat="#,##0.00";
            wsheet.Range["E3", "E9999"].NumberFormat = "#,##0.00";
            wsheet.Range["D3", "D9999"].NumberFormat = "#,##0.00";
            wsheet.Range["G3", "G9999"].NumberFormat = "#,##0.00";


            for (int i = 0; i < this.dataGridView4.Columns.Count; i++)
            {
                wsheet.Cells[3, i + 1] = this.dataGridView4.Columns[i].HeaderText;

            }
            for (int i = 0; i < this.dataGridView4.Rows.Count; i++)
            {
                DataGridViewRow row = this.dataGridView4.Rows[i];
                for (int j = 0; j < row.Cells.Count; j++)
                {
                    DataGridViewCell cell = row.Cells[j];
                    try
                    {
                        wsheet.Cells[i + 4, j + 1] = (cell.Value == null) ? "" : cell.Value.ToString();
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message);
                    }
                }
            }
            
        }

wsheet.Range["C" +(string)(3+this.dataGridView4.Rows.Count+1)].Formula = "Sum(C3:C" + (string)(3+this.dataGridView4.Rows.Count)  + ")"