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

using namespace std;

int capacitate[1001][1001], n; 
vector<int> graf[1001]; 
int tata[1001], viz[1001];

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

int BFS()
{
    queue<int> q;
    memset(tata, 0, sizeof(tata));
    memset(viz, 0, sizeof(viz));

    q.push(0);
    viz[0] = 1;
    tata[0] = -1;

    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        if (u == 2 * n + 1)
            return 1;
        for (int i = 0; i < graf[u].size(); i++)
        {
            int v = graf[u][i];
            if (capacitate[u][v] > 0 && !viz[v])
            {
                tata[v] = u;
                viz[v] = 1;
                q.push(v);
            }
        }
    }
    return 0;
}
int EdmondsKarp()
{   
    int fluxMax = 0, fluxMin, curent, adiacent;
    while (BFS())
    {
        for (int i = 0; i < graf[2 * n + 1].size(); i++)
        {
            curent = graf[2 * n + 1][i];
            if (capacitate[curent][2 * n + 1] > 0 && viz[curent])
            {
                fluxMin = capacitate[curent][2 * n + 1];
                while (tata[curent] != -1)
                {
                    adiacent = tata[curent];
                    fluxMin = min(fluxMin, capacitate[adiacent][curent]);
                    curent = adiacent;
                }
                fluxMax += fluxMin;
                curent = graf[2 * n + 1][i];
                capacitate[curent][2 * n + 1] -= fluxMin;
                capacitate[2 * n + 1][curent] += fluxMin;
                while (tata[curent] != -1)
                {
                    adiacent = tata[curent];
                    capacitate[adiacent][curent] = capacitate[adiacent][curent] - fluxMin;
                    capacitate[curent][adiacent] += capacitate[curent][adiacent] + fluxMin;
                    curent = adiacent;
                }
            }
        }
    }
    return fluxMax;
}

int main()
{
    int x, y;
    fin >> n;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            if (i != j)
            {
                graf[i].push_back(j + n);
                graf[j + n].push_back(i);
                capacitate[i][j + n] = 1;
            }
    for (int i = 1; i <= n; i++)
    {
        fin >> x >> y;
        graf[0].push_back(i);
        graf[i].push_back(0);
        capacitate[0][i] = x;
        graf[2 * n + 1].push_back(i + n);
        graf[i + n].push_back(2 * n + 1);
        capacitate[i + n][2 * n + 1] = y;
    }

    fout << EdmondsKarp() << "\n";

    for (int i = 1; i <= n; i++)
        for (int j = n + 1; j <= 2 * n; j++)
            if (capacitate[j][i] == 1)
                fout << i << " " << j - n << "\n";
    return 0;
}