ช่วยดู Error ให้ทีครับ มันบอกว่า Input String was not in a correct format

#region คืนค่า Stock ที่ยกเลิก
        private void recoverInStock()
        {
            string sql2; string IDProduct = ""; string nProduct = ""; string Bill_No = "";
            string Amount = ""; string InStock = ""; int quanboxpage; int Stock; int unitResult;
            string constr = Properties.Settings.Default.DatabaseCarserviceConnectionString;
            OleDbConnection conn = new OleDbConnection(constr);           
            sql2 = "Select [Bill_Detail].Bill_No ,[Product].Pro_ID ,[Product].Pro_Name_Model ,[Product].InStock ,[Bill_Detail].Amount From [Product] INNER Join [Bill_Detail] ON [Product].Pro_ID = [Bill_Detail].Pro_ID Where [Bill_Detail].Bill_No = '" + txtBillNo + "'";
            OleDbCommand cmd = new OleDbCommand(sql2, conn);
            try
            {
                this.Cursor = Cursors.WaitCursor;
                conn.Open();
                OleDbDataReader render = cmd.ExecuteReader();
                while (render.Read())
                {
                    Bill_No = render.GetValue(0).ToString();
                    IDProduct = render.GetValue(1).ToString();
                    nProduct = render.GetValue(2).ToString();
                    InStock = render.GetValue(3).ToString();
                    Amount = render.GetValue(4).ToString();
                }
                conn.Close();
                this.Cursor = Cursors.Default;
                txtProID.Text = IDProduct;
                cmbProName.Text = nProduct;
                quanboxpage = Convert.ToInt32(txtAmount.Text);
                Stock = Convert.ToInt32(InStock);
                unitResult = quanboxpage + Stock;
                sql2 = "UPDATE Product Set InStock = '" + unitResult + "' Where Pro_ID = '" + IDProduct + "'";
                OleDbDataAdapter adapter = new OleDbDataAdapter(sql2, conn);
                DataSet data = new DataSet();
                adapter.Fill(data, "Product");
                data.Clear();
                MessageBox.Show("สินค้าได้คืนค่า Stock เป็น" + unitResult);
            }
            catch (Exception exc)
            {
                MessageBox.Show(exc.Message, "Exception");
                this.Cursor = Cursors.Default;
            }
        }
#endregion