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);
}
}