friend BigInteger SumWithOneSign(const BigInteger& l, const BigInteger& r) {
        std::vector<int> result;
        size_t i = l.size_ - 1;
        size_t j = r.size_ - 1;
        int rest = 0;
        while (i >= 0 && j >= 0) {
            result.push_back((rest + l.digit_[i] + r.digit_[j]) % number_system);
            rest = (rest + l.digit_[i] + r.digit_[j]) / number_system;
            --i;
            --j;
        }
        if (i == 0) {
            while (j >= 0) {
                result.push_back((rest + r.digit_[j]) % number_system);
                rest = (rest + r.digit_[j]) / number_system;
            }
        } else {
            while (i >= 0) {
                result.push_back((rest + l.digit_[i]) % number_system);
                rest = (rest + l.digit_[i]) / number_system;
            }
        }
        auto first = result.begin();
        auto last = result.end();
        while (first != last && first != --last) {
            std::swap(*first++, *last);
        }
        return BigInteger(result, l.sign_);
        BigInteger result;
        const auto& max = std::max(l, r);
        const auto& min = std::min(l, r);
        const int max_size = static_cast<int>(max.digit_.size());
        const int min_size = static_cast<int>(min.digit_.size());
        for (int i = 0; i < min_size; ++i) {
            result.digit_.push_back(l.digit_[i] + r.digit_[i]);
        }
        for (int i = min_size; i < max_size; ++i) {
            result.digit_.push_back(max.digit_[i]);
        }
        result.Normalize();
        return result;
    }
    friend BigInteger SumWithDifferentSign(const BigInteger& l, const BigInteger& r) {
        /*const auto& max = std::max(l, r);
        const auto& min = std::min(l, r);
        std::vector<int> result;
        size_t i = max.size_ - 1;
        size_t j = min.size_ - 1;
        int rest = 0;
        while (j >= 0) {
            result.push_back((max.digit_[i] - min.digit_[j] + rest + number_system) % number_system);
            rest = (max.digit_[i] + rest < min.digit_[j]) ? -1 : 0;
            --i;
            --j;
        }
        if (i != 0) {
            result.push_back((max.digit_[i] + rest + number_system) % number_system);
            rest = (max.digit_[i] + rest < 0) ? -1 : 0;
        }
        while (result.back() == 0) {
            result.pop_back();
        }
        if (result.empty()) {
            result.push_back(0);
        }
        auto first = result.begin();
        auto last = result.end();
        while (first != last && first != --last) {
            std::swap(*first++, *last);
        }
        return BigInteger(result, max.sign_);
        BigInteger result;
        const auto& max = std::max(l, r);
        const auto& min = std::min(l, r);
        const int max_size = static_cast<int>(max.digit_.size());
        const int min_size = static_cast<int>(min.digit_.size());
        for (int i = 0; i < min_size; ++i) {
            result.digit_.push_back(max.digit_[i] - min.digit_[i]);
        }
        for (int i = min_size; i < max_size; ++i) {
            result.digit_.push_back(max.digit_[i]);
        }
        result.Normalize();
        return result;

    }
    friend BigInteger Multiplication(const BigInteger& l, const BigInteger& r) {
        BigInteger result;
        size_t i = l.size_ - 1;
        size_t j = r.size_ - 1;
    }