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

using namespace std;
ifstream in("harta.in");
ofstream out("harta.out");


const int MAXN = 202; 
int capacity[MAXN][MAXN]; 
int flow[MAXN][MAXN]; 
int parent[MAXN]; 
bool viz[MAXN]; 
int N; 

bool bfs(int s, int t)
{
    memset(viz, 0, sizeof(viz)); 
    queue<int> q;
    q.push(s);
    viz[s] = true;
    parent[s] = -1;

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

        for (int v = 0; v < 2 * N + 2; ++v)
        {
            // muchie neviz cu capacitate disponibila
            if (!viz[v] && capacity[u][v] - flow[u][v] > 0)
            {
                q.push(v);
                parent[v] = u;
                viz[v] = true;
            }
        }
    }

    return viz[t]; // true daca gasim calea pan ala scurgere
}

int fordFulkerson(int s, int t)
{
    int max_flow = 0;

    while (bfs(s, t))
    {
        int path_flow = INT_MAX;
        for (int v = t; v != s; v = parent[v])
        {
            int u = parent[v];
            path_flow = min(path_flow, capacity[u][v] - flow[u][v]);
        }

        // Actualizeaza fluxul pe calea gasita
        for (int v = t; v != s; v = parent[v])
        {
            int u = parent[v];
            flow[u][v] += path_flow;
            flow[v][u] -= path_flow;
        }

        max_flow += path_flow; // Adauga la fluxul maxim
    }

    return max_flow; 
}

int main()
{
    

    in >> N; 
    int total_in_deg = 0, total_out_deg = 0;
    vector<pair<int, int>> cities(N); 

    for (int i = 0; i < N; ++i)
    {
        in >> cities[i].first >> cities[i].second;
        total_out_deg += cities[i].first;
        total_in_deg += cities[i].second;
    }

    int source = 2 * N, sink = 2 * N + 1; 

   
    for (int i = 0; i < N; ++i)
    {
        capacity[source][i] = cities[i].first; 
        capacity[i + N][sink] = cities[i].second; // setam capacitatile

        for (int j = 0; j < N; ++j)
        {

            if (i != j)
            {
                capacity[i][j + N] = 1;  // muchii intre orase cu cap 1
            }
        }
    }

    int max_flow = fordFulkerson(source, sink);

    out << max_flow << endl;
    for (int i = 0; i < N; ++i)
    {
        for (int j = 0; j < N; ++j)
        {
            if (flow[i][j + N] > 0)
            {
                out << i + 1 << " " << j + 1 << endl; 
            }
        }
    }


    return 0;
}