#include <fstream>
#include <algorithm>
#include <vector>

using namespace std;

const int NMAX = 100;

pair<int, int> intrare[1 + NMAX];
pair<int, int> iesire[1 + NMAX];

vector<pair<int, int>> sol;

int main()
{
    ifstream in("harta.in");
    ofstream out("harta.out");

    ios_base::sync_with_stdio(false);
    in.tie(nullptr);
    out.tie(nullptr);

    int n;
    in >> n;

    for (int i = 1; i <= n; ++i)
    {
        in >> intrare[i].first >> iesire[i].second;

        intrare[i].second = i;
        iesire[i].second = i;
    }

    sort(intrare + 1, intrare + 1 + n);
    sort(iesire + 1, iesire + 1 + n);

    while (intrare[n].first > 0)
    {
        for (int i = n; i >= 1 && intrare[n].first > 0; --i)
        {
            if (iesire[i].second != intrare[n].second)
            {
                sol.emplace_back(make_pair(iesire[i].second, intrare[n].second));
                --intrare[n].first;
                --iesire[i].first;
            }
        }

        sort(intrare + 1, intrare + 1 + n);
        sort(iesire + 1, iesire + 1 + n);
    }

    out << sol.size() << '\n';

    for (int i = 0; i < sol.size(); ++i)
        out << sol[i].first << ' ' << sol[i].second << '\n';

    in.close();
    out.close();

    return 0;
}