private void dataGridView1_CellValidating(object sender, DataGridViewCellValidatingEventArgs e)
{
    DataGridViewTextBoxCell cell = dataGridView1[2, e.RowIndex] as DataGridViewTextBoxCell;

    if (cell != null)
    {
        if (e.ColumnIndex == 4)
        {
            char[] chars = e.FormattedValue.ToString().ToCharArray();
            foreach (char c in chars)
            {
                if (char.IsDigit(c) == false)
                {
                    MessageBox.Show("You have to enter digits only");

                    e.Cancel = true;
                    break;
                }
            }
        }
    }


}