def get_not_downloaded_isbns(all_isbns, downloaded_isbns):
    """
    Identifiy ISBNs, which haven't been downloaded
    @params:
        all_isbns   - Required  : all isbns (pd.Series)
        all_isbns   - Required  : already downlaoded isbns (pd.Series)
    @returns:
        to_download : isbns to download (list)
    """
    all_isbns = all_isbns.to_list()
    downloaded_isbns = downloaded_isbns.to_list()
    to_download = list(set(all_isbns).difference(downloaded_isbns))
    to_download.sort()
    return to_download