private void MinusSL()
        {
            int position = dgv_BillInfo.Rows.Count - 1;

            for (int i = 0; i < position; i++)
            {
                int sl = int.Parse(dgv_BillInfo.Rows[i].Cells[2].Value.ToString());
                good.UpSL(cmb_Goods.SelectedValue.ToString().Trim(), good.SL(cmb_Goods.SelectedValue.ToString().Trim()) - sl);
            }
        }