import re


def extract_page_nr(some_string):
    """ extracts the page number from a string like `Seite 21`

    :param some_string: e.g. `Seite 21`
    :type some_string: str

    :return: The page number e.g. `21`
    :rtype: str
    """

    page_nr = re.findall(r'\d+', some_string)
    if len(page_nr) > 0:
        return "-".join(page_nr)
    else:
        return some_string