restar dos textbox del datagridview

private void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)
    {
     
        if (e.ColumnIndex == 2 || e.ColumnIndex == 3)
        {
            int rowIndex = e.RowIndex;
            DataGridViewRow row = dataGridView1.Rows[rowIndex];
            int a = Convert.ToInt32(row.Cells[2].Value);
            int b = Convert.ToInt32(row.Cells[3].Value);
            row.Cells[4].Value = a + b;
        }
    }