#include <bits/stdc++.h>
using namespace std;
//infoarena taramul nicaieri
int main() {
    ifstream fin("harta.in");
    ofstream fout("harta.out");
    int n;
    fin>>n;
    vector<pair<int, int>> grade;
    int x, y;
    for(int i = 0; i < n; i++)
    {
        fin>>x>>y;
        grade.emplace_back(x, y);
    }
    vector<pair<int, int>> drumuri;
    for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                if (i!=j && grade[j].second){
                    grade[i].first--;
                    grade[j].second--;
                    drumuri.emplace_back(i + 1, j + 1);
                    if(grade[i].first == 0) break;
                }
            }
    }
    fout<<drumuri.size()<<'\n';
    for(auto &drum : drumuri)
        fout<<drum.first<<' '<<drum.second<<'\n';
    return 0;
}