def get_bank_name(account_number):
    """
    Returns the bank name based on the account number
    @param account_number: a Finnish bank account number (BBAN).
    @return: Bank name or 'Tuntematon'.
    """
    banks = {'1': u'Nordea', '2': u'Nordea', '31': u'Handelsbanken', '33':
        u'SEB', '34': u'Danske Bank', '36': u'Tapiola', '37': u'DnB NOR',
        '38': u'Swedbank', '39': u'S-Pankki', '4': u'Säästöpankki', '47':
        u'Pop', '5': u'Osuuspankki', '6': u'Ålandsbanken', '8': u'Sampo'}
    if account_number == '500001':
        return 'OKO pankki'
    elif account_number[0:4] in ('4055', '4050', '4970'):
        return 'Aktia'
    elif account_number[0:2] in banks:
        return banks[account_number[0:2]]
    elif account_number[0] in banks:
        return banks[account_number[0]]
    else:
        return u'Tuntematon'
