def display(cnpj):
    """
    Will format an adequately formatted numbers-only CNPJ string,
    adding in standard formatting visual aid symbols for display.
    """
    if not cnpj.isdigit() or len(cnpj) != 14 or len(set(cnpj)) == 1:
        return None
    return '{}.{}.{}/{}-{}'.format(cnpj[:2], cnpj[2:5], cnpj[5:8], cnpj[8:
        12], cnpj[12:])
