private void btnThongKe_Click(object sender, EventArgs e)
{
dgvDoanhThuBanVe.DataSource = tkbus.DoanhThuBanVe(dateNgayBan1.Value.Date.ToString(), dateNgayBan2.Value.Date.ToString());
int index = dgvDoanhThuBanVe.Rows.Count;
int thanhtien = 0;
for (int i = 0; i < index - 1; i++)
{
thanhtien += int.Parse(dgvDoanhThuBanVe.Rows[i].Cells["TongTien"].Value.ToString());
}
lblTongTien.Text = thanhtien.ToString();
}