main.cpp

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

struct sort_pred {
    bool operator()(const std::pair<int, int> &left, const std::pair<int, int> &right) {
        return left.first == right.first ? left.second < right.second : left.first < right.first;
    }
};


int main() {
    while (true) {
        int cityCount;
        cin >> cityCount;
        if (cityCount == 0) break;

        vector<pair<int, int> > cities;

        for (int i = 0; i < cityCount; i++) {
            int inv, earn;
            cin >> inv >> earn;
            cities.push_back(make_pair(inv, earn));
        }

        sort(cities.begin(), cities.end(), sort_pred());

        bool done = false;
        for (std::vector<pair<int, int> >::iterator i = cities.begin(); i != cities.end();i++) {
            auto next = i;
            next++;
            if (next != cities.end() && next->first > i->first && next->second <= i->second) {
                cout << "NO\n";
                done = true;
                break;
            }
        }
        if (!done) cout << "SI\n";
    }
    return 0;
}