#include <fstream>
#include <iostream>
#include <climits>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;
ifstream fin("harta.in");
ofstream fout("harta.out");

struct edge{
    int y;
    int capacity;
    int flow;
};

int n, x, y;

queue<int> q;
int father[2005];
vector<edge> ad[2005];
int source, destination;
bool explored[2005];

bool cmp(edge e1, edge e2) {
    return e1.y < e2.y;
}


int bfs() {

    fill(father + 1, father + 2 * n + 3, 0);
    fill(explored + 1, explored + 2 * n + 3, 0);

    explored[source] = 1;
    q.push(source);

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

        q.pop();

        if(node == destination)
            continue;

        for(auto vec : ad[node]) {
            if(vec.flow < vec.capacity && explored[vec.y] == 0) {
                explored[vec.y] = 1;
                q.push(vec.y);
                father[vec.y] = node;
            }
        }
    }

    if(explored[destination] == 0)
        return 0; // destination can't be reached!
    else
        return 1; // destination reached!
}

int findPosition(int node, int vec){

    int st = 0;
    int dr = ad[node].size() - 1;
    int mij;
    while(st <= dr) {
        mij = (st + dr) / 2;
        if(ad[node][mij].y == vec)
            return mij;
        else if(ad[node][mij].y < vec)
            st = mij + 1;
        else if(ad[node][mij].y > vec)
            dr = mij - 1;
    }


    return -1;
};

int main() {
    fin >> n;

    source = 2 * n + 2; // aleg un nod sursa
    destination = 2 * n + 1; // aleg un nod destinatie

    // pun muchiile citite in problema cu capacitate 1
    for (int i = 1; i <= n; i++)
    {
        fin >> x >> y; // x pleaca; y intra

        // adaug muchii de capacitate y de la sursa la primul set de noduri
        ad[source].push_back({i, y, 0});
        ad[i].push_back({source, 0, 0});

        // adaug muchii de capacitate x de la al doilea set de noduri la destinatie
        ad[i + n].push_back({destination, x, 0});
        ad[destination].push_back({i + n, 0, 0});
    }

    for (int i = 1; i <= n ; i++)
        for(int j = n + 1; j <= 2 * n; j ++) {
            if(i != j - n) {
                ad[i].push_back({j, 1, 0});
                ad[j].push_back({i, 0, 0});
            }
        }
    // sortez listele de adiacenta crescator
    for(int i = 1 ; i <= 2 * n + 2; i ++)
        sort(ad[i].begin(), ad[i].end(), cmp);

//    for(int i = 1; i <= 2 * n + 2; i ++) {
//        cout << i << ": ";
//        for(auto j : ad[i])
//            cout << j.y << " ";
//        cout << '\n';
//    }
    int max_flow = 0;

    while(bfs()) {

        for(auto vec : ad[destination]) {
            int min_flow = INT_MAX;

            int pos = findPosition(vec.y, destination);
            if (explored[vec.y] == 0 || ad[vec.y][pos].flow == ad[vec.y][pos].capacity)
                continue;

            int node = destination;
            father[destination] = vec.y;

            while (father[node] != 0) {
                int pos1 = findPosition(father[node], node);

                min_flow = min(min_flow, ad[father[node]][pos1].capacity - ad[father[node]][pos1].flow);
                node = father[node];
            }

            node = destination;
            max_flow += min_flow;

            while (father[node] != 0) {
                int pos1 = findPosition(father[node], node);
                ad[father[node]][pos1].flow += min_flow;

                int pos2 = findPosition(node, father[node]);
                ad[node][pos2].flow -= min_flow;
                node = father[node];
            }
        }
    }

    fout << max_flow << '\n';

    for(int i = 1; i <= n ; i ++)
        for(auto j : ad[i]) {
            if(j.flow == 1)
                fout << i << " " << j.y - n << '\n';
        }
    return 0;
}