#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

int main() {
    ifstream fin("harta.in");
    ofstream fout("harta.out");
    
    int N;
    fin >> N;
    vector<pair<int, int>> cities;
    for (int i = 0; i < N; ++i) {
        int x, y;
        fin >> x >> y;
        cities.push_back(make_pair(x, y));
    }

    vector<pair<int, int>> roads;
    for (int i = 0; i < N; ++i) {
        for (int j = 1; j <= cities[i].first; ++j) {
            roads.push_back(make_pair(i + 1, i + j + 1));
        }
    }

    fout << roads.size() << "\n";
    for (const auto& road : roads) {
        fout << road.first << " " << road.second << "\n";
    }

    fin.close();
    fout.close();

    return 0;
}