def get_chapter_id(url: str=None) ->str:
    """
    Returns the ID number from a given MangaDex chapter URL.

    Parameters:
        url (str): MangaDex chapter URL

    Returns:
        str: MangaDex chapter ID number
    """
    if url is None or 'mangadex.' not in url or '/chapter/' not in url:
        return ''
    start = url.index('/chapter/')
    id = url[start:]
    return str(id)
