#include <bits/stdc++.h>
using namespace std;

struct Food {
    int S, M, id;
    bool operator<(const Food &other) const {
        if (S != other.S) return S > other.S;
        return M < other.M; 
    }
};

int main() {
    int N;
    cin >> N;
    vector<Food> foods;
    for (int i = 1; i <= N; ++i) {
        int S, M;
        cin >> S >> M;
        if (M >= 4) {
            foods.push_back({S, M, i});
        }
    }

    priority_queue<Food> pq;
    for (auto f : foods) {
        pq.push(f);
    }

    vector<pair<int,int>> days;
    int day = 1;

    while (pq.size() >= 2) {
        Food first = pq.top(); pq.pop();
        Food second = pq.top(); pq.pop();

        if (first.S >= day && second.S >= day && first.M + second.M >= 9) {
            days.push_back({first.id, second.id});
            day++;
        } else {
            pq.push(first);
            pq.push(second);
            break;
        }
    }

    cout << days.size() << "\n";
    for (auto &d : days) {
        cout << d.first << " " << d.second << "\n";
    }

    return 0;
}