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

using namespace std;

class flow
{
public:
    flow(int n, int source, int dest)
    {
        this->source = source;
        this->dest = dest;
        this->n = n;
        cap.resize(n + 1, vector<int>(n + 1));
        flux.resize(n + 1, vector<int>(n + 1));
        vecini.resize(n + 1);
        viz.resize(n + 1);
        father.resize(n + 1);
    }
    void AddEdge(int x, int y, int c)
    {
        if(cap[x][y] == 0 && cap[y][x] == 0)
        {
            vecini[x].push_back(y);
            vecini[y].push_back(x);
        }
        cap[x][y] += c;
    }
    int GetMaxFlow()
    {
        int ret = 0;
        BFS();
        while(viz[dest])
        {
            for(auto nod:vecini[dest])
            {
                if(cap[nod][dest] == flux[nod][dest] || viz[nod] == false)
                    continue;
                father[dest] = nod;

                int mn = cap[father[dest]][dest] - flux[father[dest]][dest];
                nod = father[dest];
                while(nod != source)
                {
                    mn = min(mn, cap[father[nod]][nod] - flux[father[nod]][nod]);
                    nod = father[nod];
                }

                nod = dest;
                while(nod != source)
                {
                    flux[father[nod]][nod] += mn;
                    flux[nod][father[nod]] -= mn;
                    nod = father[nod];
                }

                ret += mn;
            }
            BFS();
        }
        return ret;
    }
    void GetMinCut(vector<pair<int, int> > &ret)
    {
        ret.reserve(GetMaxFlow());
        for(int i = 1; i <= n; ++i)
            for(auto j:vecini[i])
                if(viz[i] == true && viz[j] == false)
                    ret.push_back({i, j});
    }
    vector<vector<int> > flux;
private:
    void BFS()
    {
        fill(viz.begin(), viz.end(), false);
        queue<int> q;
        q.push(source);
        viz[source] = true;

        while(q.empty() == false)
        {
            int nod = q.front();
            q.pop();
            if(nod == dest)
                continue;

            for(auto v:vecini[nod])
            {
                if(cap[nod][v] == flux[nod][v] || viz[v])
                    continue;
                viz[v] = true;
                q.push(v);
                father[v] = nod;
            }
        }
    }
    int source, dest;
    int n;
    vector<bool> viz;
    vector<vector<int> > vecini;
    vector<vector<int> > cap;
    vector<int> father;
};

int main()
{
    ifstream in("harta.in");
    int n;
    in >> n;
    int source = 2 * n + 1, dest = 2 * n + 2;
    flow gr(dest, source, dest);
    for(int i = 1; i <= n; ++i)
        for(int j = n+1; j <= 2 * n; ++j)
            if(j != n+i)
                gr.AddEdge(i, j, 1);
    int x, y;
    for(int i = 1; i <= n; ++i)
    {
        in >> x >> y;
        gr.AddEdge(source, i, x);
        gr.AddEdge(n+i, dest, y);
    }
    in.close();

    ofstream out("harta.out");
    out << gr.GetMaxFlow() << "\n";
    for(int i = 1; i <= n; ++i)
        for(int j = 1; j <= n; ++j)
            if(gr.flux[i][n+j] != 0)
                out << i << " " << j << "\n";
    out.close();
    return 0;
}