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

using namespace std;

const int MAX = 205;
int capacity[MAX][MAX]; 
int bfs(int s, int t, vector<int>& parent, vector<vector<int>>& adj) {
    fill(parent.begin(), parent.end(), -1);
    parent[s] = -2;
    queue<pair<int, int>> q;
    q.push({s, INT_MAX});

    while (!q.empty()) {
        int cur = q.front().first;
        int flow = q.front().second;
        q.pop();

        for (int next : adj[cur]) {
            if (parent[next] == -1 && capacity[cur][next]) {
                parent[next] = cur;
                int new_flow = min(flow, capacity[cur][next]);
                if (next == t)
                    return new_flow;
                q.push({next, new_flow});
            }
        }
    }

    return 0;
}

int maxflow(int s, int t, vector<vector<int>>& adj) {
    int flow = 0;
    vector<int> parent(MAX);
    int new_flow;

    while (new_flow = bfs(s, t, parent, adj)) {
        flow += new_flow;
        int cur = t;
        while (cur != s) {
            int prev = parent[cur];
            capacity[prev][cur] -= new_flow;
            capacity[cur][prev] += new_flow;
            cur = prev;
        }
    }

    return flow;
}

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

    int n;
    fin >> n;
    vector<vector<int>> adj(MAX);
    int s = 0, t = 2 * n + 1; 

    for (int i = 1; i <= n; ++i) {
        int out, in;
        fin >> out >> in;
        capacity[s][i] = out;
        capacity[i + n][t] = in;
        adj[s].push_back(i);
        adj[i + n].push_back(t);
        for (int j = 1; j <= n; ++j) {
            if (i != j) {
                capacity[i][j + n] = 1; 
                adj[i].push_back(j + n);
                adj[j + n].push_back(i);
            }
        }
    }

    int total_flow = maxflow(s, t, adj);

    fout << total_flow << endl;
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= n; ++j) {
            if (i != j && capacity[j + n][i] > 0) {
                fout << i << " " << j << endl;
            }
        }
    }

    fin.close();
    fout.close();
    return 0;
}