// UUID: 6880f8ea-3ec5-44ea-ba4c-08e30d5f5101
#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

struct Etel {
    int id;
    int szav;
};

bool hasonlit(const Etel& a, const Etel& b) {
    return a.szav < b.szav;
}

vector<Etel> negyesek_alap;
vector<Etel> otosok_alap;
vector<pair<int, int>> vegso_parok;

bool check(int K, bool mentes) {
    vector<Etel> pool_4;
    vector<Etel> pool_5;
    
    int idx_4 = negyesek_alap.size() - 1;
    int idx_5 = otosok_alap.size() - 1;

    if (mentes) vegso_parok.clear();

    for (int nap = K; nap >= 1; --nap) {
        while (idx_4 >= 0 && negyesek_alap[idx_4].szav >= nap) {
            pool_4.push_back(negyesek_alap[idx_4]);
            idx_4--;
        }
        while (idx_5 >= 0 && otosok_alap[idx_5].szav >= nap) {
            pool_5.push_back(otosok_alap[idx_5]);
            idx_5--;
        }

        if (!pool_4.empty() && !pool_5.empty()) {
            if (mentes) {
                vegso_parok.push_back({pool_4.back().id, pool_5.back().id});
            }
            pool_4.pop_back();
            pool_5.pop_back();
        } 
        else if (pool_5.size() >= 2) {
            if (mentes) {
                int e1 = pool_5.back().id;
                pool_5.pop_back();
                int e2 = pool_5.back().id;
                pool_5.pop_back();
                vegso_parok.push_back({e1, e2});
            } else {
                pool_5.pop_back();
                pool_5.pop_back();
            }
        } 
        else {
            return false;
        }
    }
    return true;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N;
    if (!(cin >> N)) return 0;

    for (int i = 1; i <= N; ++i) {
        int s, m;
        cin >> s >> m;
        if (m == 4) {
            negyesek_alap.push_back({i, s});
        } else if (m >= 5) {
            otosok_alap.push_back({i, s});
        }
    }

    sort(negyesek_alap.begin(), negyesek_alap.end(), hasonlit);
    sort(otosok_alap.begin(), otosok_alap.end(), hasonlit);

    int bal = 0;
    int jobb = N / 2;
    int megoldas = 0;

    while (bal <= jobb) {
        int kozep = bal + (jobb - bal) / 2;
        if (check(kozep, false)) {
            megoldas = kozep;
            bal = kozep + 1;
        } else {
            jobb = kozep - 1;
        }
    }

    cout << megoldas << "\n";
    
    if (megoldas > 0) {
        check(megoldas, true);
        for (int i = vegso_parok.size() - 1; i >= 0; i--) {
            cout << vegso_parok[i].first << " " << vegso_parok[i].second << "\n";
        }
    }

    return 0;
}