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

using namespace std;

int bfs(int s, int t, vector<int>& tata, vector<vector<int>>& muchii, 
            vector<vector<int>>& cap, vector<vector<int>>& flux){
    for(int i = 0; i < tata.size(); i++)
        tata[i] = -1;

    int f = INT_MAX;

    tata[s] = s;
    queue<int> q;
    q.push(s);

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

        for(int i = 0; i < muchii[x].size(); i++){
            int vecin = muchii[x][i];

            if(tata[vecin] == -1 && flux[x][vecin] < cap[x][vecin]){
                tata[vecin] = x;

                if(f > cap[x][vecin] - flux[x][vecin])
                    f = cap[x][vecin] - flux[x][vecin];

                if(t == vecin)
                    return f;
                
                q.push(vecin);
            }
        }
    }
    
    return 0;
}

int main(){
    ifstream f("harta.in");
    ofstream g("harta.out");

    int n;
    f >> n;

    int s = 0, t = 2*n+1;

    vector<int> tata(t+1, -1);
    vector<vector<int>> muchii(t+1);
    vector<vector<int>> flux(t+1, vector<int>(t+1, 0));
    vector<vector<int>> capacitate(t+1, vector<int>(t+1, 0));

    for(int i = 1; i <= n; i++){
        int x, y;
        f >> x >> y;

        muchii[i].push_back(s);
        muchii[s].push_back(i);
        capacitate[s][i] = x;

        for(int j = 1; j <= n; j++)
            if(i != j){
                muchii[i].push_back(j+n);
                muchii[j+n].push_back(i);
                capacitate[i][j+n] = 1;
            }

        muchii[i+n].push_back(t);
        muchii[t].push_back(i+n);
        capacitate[i+n][t] = y;
    }

    int nrMuchii = 0;
    while(1){
        int f = bfs(s, t, tata, muchii, capacitate, flux);
        
        if(f){
            nrMuchii += f;

            int x = t;
            while(x != s){
                int y = tata[x];
                flux[y][x] += f;
                flux[x][y] -= f;
                x = y;
            }
        }
        else
            break;
    }

    g << nrMuchii << '\n';
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= n; j++)
            if(flux[i][j+n])
                g << i << ' ' << j << '\n';

    return 0;
}