#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#include <climits>
using namespace std;

ifstream fin("harta.in");
ofstream fout("harta.out");

int N, SURSA, DESTINATIE;
int m_a[301][301];
vector<bool> vizitat;
vector<int> v_p;

int BFS()
{
    v_p.clear();
    v_p.resize(DESTINATIE + 1, 0);
    vizitat.clear();
    vizitat.resize(DESTINATIE + 1, false);

    v_p[SURSA] = -1;
    vizitat[SURSA] = true;


    queue <int> q;
    q.push(SURSA);

    while (!q.empty())
    {
        int n_curent = q.front();
        q.pop();

        if (n_curent == DESTINATIE) return 1;

        for (int i = 1; i <= DESTINATIE; i++)
            if (!vizitat[i] && m_a[n_curent][i] > 0)
            {
                q.push(i);
                v_p[i] = n_curent;
                vizitat[i] = true;
            }
    }
    return 0;
}

int FORDFULKERSON()
{
    int m_flux = 0;
    while (BFS())
    {
        for(int i = 1; i <= N; i++)
        {
            if (vizitat[N + i] && m_a[N + i][DESTINATIE] > 0)
            {
                v_p[DESTINATIE] = N + i;

                int flux = INT_MAX;
                for(int nod = DESTINATIE; nod != SURSA; nod = v_p[nod])
                    flux = min(flux, m_a[v_p[nod]][nod]);

                for (int nod = DESTINATIE; nod != SURSA; nod = v_p[nod])
                {
                    m_a[v_p[nod]][nod] -= flux;
                    m_a[nod][v_p[nod]] += flux;
                }
                m_flux += flux;
            }
        }
    }
    return m_flux;
}

int main ()
{
    fin>>N;

    SURSA = 0;
    DESTINATIE = 2 * N + 1;

    for(int i = 1; i <= N; i++)
    {
        int x, y;
        fin >> y >> x;

        m_a[SURSA][i] = y;
        m_a[i + N][DESTINATIE] = x;

        for(int j = 1; j <= N; j++)
        {
            if(i != j)
            {
                m_a[i][j + N] = 1;
            }
        }
    }

    fout<<FORDFULKERSON()<<endl;

    for(int i = 1; i <= N; i++)
    {
        for(int j = 1; j <= N; j++)
        {
            if(m_a[i][j + N] == 0 && i != j)
            {
                fout << i << " " << j << endl;
            }
        }
    }

    return 0;
}