private void BtnDodaj_Click(object sender, EventArgs e)//Dodaj means Add in Croatian
        {
            if (DTPVracanje.Checked == false)//DTPvracanje is a datetimepicker
            {
                for (int Row = 0; Row < DataGridView.Rows.Count;  Row++)
                {
                    if (DataGridView.Rows[Row].Cells[1].Value.ToString() == CboSifraKnjige.Text &&
                        DataGridView.Rows[Row].Cells[5].Value.Equals(Convert.IsDBNull(DTPVracanje.Value)))//column 5 contains DBnull if DTPVracanje is unchecked
                    {
                        MessageBox.Show("Knjiga je vani");//Book is out in Croatian[
                    }
                    else
                    {
                        Dodaj();
                    }
                }
            }
        }