txtThanhTien.Text = "";
            String TextTrongLuong = txtTrongLuong.Text;
            if (TextTrongLuong != "")
            {
                int TrongLuong;
                try
                {
                    TrongLuong = int.Parse(TextTrongLuong);
                    int ThanhTien = 0;
                    if (TrongLuong > 0 && TrongLuong <= 100)
                    {
                        ThanhTien = TrongLuong * 500;
                    }
                    else if (TrongLuong > 100 && TrongLuong < 130)
                    {
                        ThanhTien = (100 * 500) + ((TrongLuong - 100) * 400);
                    }
                    else
                    {
                        ThanhTien = (100 * 500) + (30 * 400) + ((TrongLuong - 130) * 300);
                    }
                    txtThanhTien.Text = ThanhTien.ToString();
                }
                catch (Exception error)
                {
                    txtThanhTien.Text = "Loi";
                }


            }