std::ostream& operator<<(std::ostream& os, const city c)
{
    switch (c)
    {
    case city::LONDON:
        os << "런던";
        return os;
    case city::MOSCOW:
        os << "모스크바";
        return os;
    case city::SEOUL:
        os << "서울";
        return os;
    case city::SEATTLE:
        os << "시애틀";
        return os;
    case city::DUBAI:
        os << "두바이";
        return os;
    case city::SYDNEY:
        os << "시드니";
        return os;
    default:
        return os;
    }
}