import json


def get_person_group_key(person_id: str, group_id: str):
    """
    Get a unique string from the person and group IDs in a deterministic way.

    :param person_id: the ID of the person
    :param group_id: the ID of the group
    :return: the derived unique key
    """
    return json.dumps([person_id, group_id])