//Auto complete the unit price unit base on box
private void txtPBox_KeyUp(object sender, KeyEventArgs e)
{
int input;
if (int.TryParse(txtPBox.Text, out input))
{
Category _cate = CategoryBUS.getById((int)cboCate.SelectedValue);
int? output = (input + _cate.Unit - 1) / _cate.Unit;
txtPUnit.Text = output.ToString();
}
}