#include <bits/stdc++.h>
using namespace std;

int S, D;
int n, m, TT[1005];
int C[1005][1005], F[1005][1005];
bool f[1005];
vector <int> v[1005];
queue <int> q;
const int INF = 2000000000;
inline bool Bfs(){
    memset(f, 0, sizeof(f));
    q.push(S);
    f[S] = 1;
    while(!q.empty()){
        int nod = q.front();
        q.pop();
        if(nod == D) continue ;
        for(auto it : v[nod]){
            if(C[nod][it] == F[nod][it] || f[it]) continue ;
            q.push(it);
            f[it] = 1;
            TT[it] = nod;
        }
    }
    return f[D];
}
int main()
{
    freopen("harta.in", "r", stdin);
    freopen("harta.out", "w", stdout);
    scanf("%d", &n);
    D = n * 2 + 1;
    int x, y;
    for(int i = 1; i <= n ; ++i){
        scanf("%d%d", &x, &y);
        v[S].push_back(i);
        v[i].push_back(S);
        v[n + i].push_back(D);
        v[D].push_back(n + i);
        C[S][i] = x; C[n + i][D] = y;
    }
    for(int i = 1; i <= n ; ++i){
        for(int j = 1; j <= n ; ++j){
            if(i == j) continue ;
            v[i].push_back(j + n);
            v[j + n].push_back(i);
            C[i][j + n] = 1;
        }
    }
    int flow;
    for(flow = 0; Bfs() ;){
        for(auto it : v[D]){
            int nod = it;
            if(F[nod][D] == C[nod][D] || !f[nod]) continue ;
            TT[D] = nod;
            int fmin = INF;
            for(nod = D; nod != S ; nod = TT[nod])
                fmin = min(fmin, C[TT[nod]][nod] - F[TT[nod]][nod]);
            if(fmin == 0) continue ;
            for(nod = D; nod != S ; nod = TT[nod]){
                F[TT[nod]][nod] += fmin;
                F[nod][TT[nod]] -= fmin;
            }
            flow += fmin;
        }
    }
    printf("%d\n", flow);
    for(int i = 1; i <= n ; ++i)
        for(int j = 1; j <= n ; ++j)
            if(F[i][j + n] == 1) printf("%d %d\n", i, j);
    return 0;
}