private void GridView_Bills_CellValueChanged(object sender, DataGridViewCellEventArgs e)
         
        {
            if (e.RowIndex > -1)
            {
                DataGridViewRow row = GridView_Bills.Rows[e.RowIndex];
                string value_Quantity = row.Cells["quantity"].Value.ToString();
                string value_Gross = row.Cells["gross"].Value.ToString();
                string value_Tara = row.Cells["tara"].Value.ToString();


                int result;

                if (Int32.TryParse(value_Quantity, out result)
                    && Int32.TryParse(value_Gross, out result)
                    && Int32.TryParse(value_Tara, out result))
                {
                    row.Cells["net"].Value = Convert.ToDecimal(value_Gross) - Convert.ToDecimal(value_Tara);
                    row.Cells["lack"].Value = Convert.ToDecimal(value_Quantity) - (Convert.ToDecimal(value_Gross) - Convert.ToDecimal(value_Tara));
                }
            }
        }