int SttTang()
        {
            int          dem   = 0;
            int          temp  = 1;
            FormNguoiDat frmND = new FormNguoiDat();

            dem = frmND.dgvNguoiDat.RowCount;
            if (dem == 0)
            {
                return(temp);
            }
            else
            {
                foreach (DataGridViewRow row in frmND.dgvNguoiDat.Rows)
                {
                    if (temp < int.Parse(row.Cells[0].Value.ToString()))
                    {
                        temp = int.Parse(row.Cells[0].Value.ToString());
                    }
                }
                temp += 1;
                return(temp);
            }
        }
        private void button4_Click(object sender, EventArgs e)
        {
            FormNguoiDat frm = new FormNguoiDat();

            frm.Show();
        }