Add a row with totals to a DataGridView

public Form1()
        {
            InitializeComponent();

            dataGridView1.ColumnCount = 3;
            dataGridView1.Columns[0].Name = "Col 1";
            dataGridView1.Columns[0].Width = 80;
            dataGridView1.Columns[1].Name = "Col 2";
            dataGridView1.Columns[1].Width = 80;
            dataGridView1.Columns[2].Name = "Col 3";
            dataGridView1.Columns[2].Width = 80;

            dataGridView1.AllowUserToAddRows = false; //removes "extra" row
            dataGridView1.RowHeadersDefaultCellStyle.Padding = new Padding(3);//helps to get rid of the selection triangle

            // add 3 rows with values
            string[] row = new string[] { "1", "2", "3" };
            dataGridView1.Rows.Add(row);
            row = new string[] { "2", "2", "4" };
            dataGridView1.Rows.Add(row);
            row = new string[] { "3", "3", "7" };
            dataGridView1.Rows.Add(row);
        }

        private void TotalColumnsBtn(object sender, EventArgs e)
        {
            int sum1 = 0, sum2 = 0;
            for (int i = 0; i < dataGridView1.RowCount; i++)
            {
                // we only sum the first and third column as an example
                sum1 += Convert.ToInt32(dataGridView1.Rows[i].Cells[0].Value);
                sum2 += Convert.ToInt32(dataGridView1.Rows[i].Cells[2].Value);
            }
            // add the total row
            string[] totalrow = new string[] { sum1.ToString(), "", sum2.ToString() };
            dataGridView1.Rows.Add(totalrow);
            // add a rowheadertitle
            dataGridView1.RowHeadersWidth = 60;
            dataGridView1.Rows[dataGridView1.RowCount - 1].HeaderCell.Value = "Total";   
        }
    }