#include<fstream>
#include<vector>
#include<queue>
#include<climits>
using namespace std;

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

int n, m, x, y, z, s, t;
int capacitate[5001][5001];
vector<vector<int>> graf;
int tata[5001];

bool bfs(){
    queue<int> q;
    int vizitat[t+1];

    for(int i=1 ; i<=t; i++)
        vizitat[i] = 0;

    q.push(s);
    vizitat[s] = 1;
    tata[s] = -1;
    while(!q.empty()){
        int u = q.front();
        q.pop();
        for(auto v: graf[u]){
            if(vizitat[v] == 0 && capacitate[u][v] != 0){
                tata[v] = u;
                if(v == t)
                    return true;

                vizitat[v] = 1;
                q.push(v);
            }
        }
    }

    return false;
}

int flux_maxim(){
    int fluxmax = 0;
    while (bfs()){
        int u, v = t, flux = INT_MAX;

        while(v!=s){
            u = tata[v];

            if(capacitate[u][v] < flux)
                flux = capacitate[u][v];

            v = tata[v];
        }

        v = t;

        while(v != s){
            u = tata[v];
            capacitate[u][v] -= flux;
            capacitate[v][u] += flux;
            v = tata[v];
        }

        fluxmax += flux;
    }

    return fluxmax;

}

int main(){
    fin>>n;
    s = 0;
    t = 2*n + 1;
    graf.resize(t+1);

    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[s].push_back(i);
        graf[i].push_back(s);
        capacitate[s][i] = x;
        graf[t].push_back(i+n);
        graf[i+n].push_back(t);
        capacitate[i+n][t] = y;
    }

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

    for(int i=1; i<=n; i++){
        for(int j=1; j<=n; j++){
            if(capacitate[j+n][i] == 1)
                fout<<i<<" "<<j<<"\n";
        }
    }
}