void multiply(std::vector<int> &a, std::vector<int> &b, std::vector<int> &res) {
        if(std::max(a.size(), b.size()) < 100) {
            simple_multiply(a, b, res);
            return;
        }
        if(a.size() < b.size()) {
            std::swap(a, b);
        }

        while(b.size() < a.size()) {
            b.push_back(0);
        }
        while((b.size() & 1)) {
            a.push_back(0);
            b.push_back(0);
        }
        int mid = a.size() / 2;
        std::vector<int> al, ar, bl, br;
        for(int i = 0; i < a.size(); i++) {
            if(i < mid) {
                al.push_back(a[i]);
                bl.push_back(b[i]);
            }
            else {
                ar.push_back(a[i]);
                br.push_back(b[i]);
            }
        }
        std::vector<int> r1, r2, r3, r4;
        std::vector<int> kl, kr;
        sum(al, ar, kl);
        sum(bl, br, kr);
        multiply(al, bl, r1);
        multiply(kl, kr, r2);
        multiply(ar, br, r4);
        reverse(r1.begin(), r1.end());
        reverse(r2.begin(), r2.end());
        reverse(r4.begin(), r4.end());
        decrease(r2, r1, r3);
        decrease(r3, r4, r2);
        for(int i = 0; i < mid; i++) {
            r2.push_back(0);
        }
        for(int i = 0; i < 2 * mid; i++) {
            r4.push_back(0);
        }
        reverse(r1.begin(), r1.end());
        reverse(r2.begin(), r2.end());
        reverse(r4.begin(), r4.end());
        sum(r1, r2, res);
        std::vector<int> res2;
        sum(res, r4, res2);
        res = res2;
    }
};