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

using namespace std;

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

int N, M;
vector <vector<int>> adj, capacity;
vector <int> parent;
vector <bool> vis;

bool BFS()
{
    for (int i = 0; i <= 2*N+1; i++)
    {
        parent[i] = 0;
        vis[i] = false;
    }
    parent[0] = -1;
    vis[0] = true;
    queue <int> q;
    q.push(0);

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

        if (current == 2 * N + 1)
            return 1;

        for (int next : adj[current])
        {
            if (!vis[next] && capacity[current][next])
            {
                parent[next] = current;
                vis[next] = true;
                q.push(next);
            }
        }
    }
    return false;
}

int MaxFlow(int start, int end)
{
    long max_flow = 0;  //initializam fluxul maxim pe care il calculam

    while (BFS()) //cat timp exista un drum prin care putem ajunge din nodul de start in nodul destinatie
    {
        for (auto x : adj[2 * N + 1])
        {
            if (capacity[x][2 * N + 1] > 0 && vis[x])
            {
                int flow = capacity[x][2 * N + 1], u, v;
                for (int node = end; node != start; node = parent[node])
                    flow = min(flow, capacity[parent[node]][node]);
                for (int node = end; node != start; node = parent[node])
                {
                    capacity[parent[node]][node] -= flow;
                    capacity[node][parent[node]] += flow;
                }

                max_flow += flow;
            }
        }
    }

    return max_flow;
}

int main()
{
    fin >> N;
    adj.resize(2 * N + 5);
    parent.resize(2 * N + 5, 0);
    vis.resize(2 * N + 5, false);
    capacity.resize(2 * N + 5, vector <int>(2 * N + 5));
    for (int i = 1; i <= N; i++)
        for (int j = 1; j <= N; j++)
            if (i != j)
            {
                adj[i].push_back(j + N);
                adj[j + N].push_back(i);
                capacity[i][j + N] = 1;
            }
    int in, out;
    for (int i = 1; i <= N; i++)
    {
        fin >> in >> out;
        adj[0].push_back(i);
        adj[i].push_back(0);
        capacity[0][i] = in;
        adj[2 * N + 1].push_back(i + N);
        adj[i + N].push_back(2 * N + 1);
        capacity[i + N][2 * N + 1] = out;
    }
    int maxFlow = MaxFlow(0, 2 * N + 1);
    fout << maxFlow << endl;
    for (int i = 1; i <= N; i++)
        for (int j = N + 1; j <= 2 * N; j++)
            if (capacity[j][i] == 1)
                fout << i << " " << j - N << '\n';
    return 0;
}