from bs4 import BeautifulSoup
import re


def parse_saldo(page_source):
    """Parses saldo.

    :param page_source: HTML page source
    :return: Saldo or None
    :rtype: str or None
    """
    soup = BeautifulSoup(page_source, 'lxml')

    lines = soup.find_all('div', class_='kontozeile_center')

    if lines:
        saldo = re.findall('-?\d+,\d{2}', lines[0].text)

        if saldo:
            return saldo[0]

    return None