specified cast is not valid

dataGridView1.Columns.Add("1","User Login Id");
     dataGridView1.Columns.Add("2", "Employee Id");
     dataGridView1.Columns.Add("3", "Serial No");
     dataGridView1.Columns.Add("4", "Enroll Date");

 public partial class frmenrolldetails : Form
       {
        DBConnect dbcon = new DBConnect();
        Timer timer1 = new Timer();
        public frmenrolldetails()
        {
            InitializeComponent();
        }

        private void frmenrolldetails_Load(object sender, EventArgs e)
        {
            timer1.Start();
            timer1.Tick +=new EventHandler(timer1_Tick);
            dataGridView1.Columns.Add("1", "User Login Id");
            dataGridView1.Columns.Add("2", "Employee Id");
            dataGridView1.Columns.Add("3", "Enroll Serial No");
            dataGridView1.Columns.Add("4", "Enroll Date");
            BindGrid();
        }

        public void BindGrid()
        {
            try
            {
                dbcon.OpenConnection();
                string str = "select ulogin_id,emp_id,enroll_sno,enroll_dt from enroll_dtls";
                OracleCommand cmd = new OracleCommand(str, DBConnect.connection);
                OracleDataReader dr = cmd.ExecuteReader();
                int rec = 0;
                if (dr.HasRows)
                {
                    while (dr.Read())
                    {
                        int n = dataGridView1.Rows.Add();
                        for (int i = 0; i < 4; i++)
                        {
                            dataGridView1.Rows[n].Cells[i].Value = dr.GetString(i);
                        }
                        rec++;
                    }
                    toolStripLabel1.Text = rec + " Records Found";
                }
                else
                {
                    toolStripLabel1.Text = "No Records Found";
                }
            }
            catch (OracleException err)
            {
                MessageBox.Show(err.Message);
            }
        }

        void timer1_Tick(object ob, EventArgs e)
        {
            toolStripLabel2.Text = DateTime.Now.ToString();
        }
    }
}