#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <vector>
#include <algorithm>
#include <queue>
using namespace std;

const int NMAX = 205, INF = 0x3f3f3f3f;

int N, From, To, Cap[NMAX][NMAX], Flow[NMAX][NMAX], Source, Sink, Father[NMAX];
bool Used[NMAX];
vector<int> G[NMAX];

bool BFS()
{
    for(int i = Source; i <= Sink; ++ i) Father[i] = Used[i] = 0;
    queue<int> Q;
    Q.push(Source);
    Used[Source] = 1;

    while(!Q.empty())
    {
        int Node = Q.front();
        Q.pop();

        if(Node == Sink) continue;

        for(vector<int> :: iterator it = G[Node].begin(); it != G[Node].end(); ++ it)
            if(Cap[Node][*it] > Flow[Node][*it] && !Used[*it])
            {
                Used[*it] = 1;
                Father[*it] = Node;
                Q.push(*it);
            }
    }

    return Used[Sink];
}

void MaxFlow()
{
    while(BFS())
    {
        for(vector<int> :: iterator it = G[Sink].begin(); it != G[Sink].end(); ++ it)
            if(Used[*it] && Cap[*it][Sink] != Flow[*it][Sink])
            {
                Father[Sink] = *it;
                int MinFlow = INF;
                for(int Node = Sink; Node != Source; Node = Father[Node])
                    MinFlow = min(MinFlow, Cap[Father[Node]][Node] - Flow[Father[Node]][Node]);
                for(int Node = Sink; Node != Source; Node = Father[Node])
                {
                    Flow[Father[Node]][Node] += MinFlow;
                    Flow[Node][Father[Node]] -= MinFlow;
                }
            }
    }

    vector<pair<int, int> > Ans;
    for(int i = 1; i <= N; ++ i)
        for(int j = N + 1; j <= 2 * N; ++ j)
            if(Flow[i][j] == 1)
                Ans.push_back(make_pair(i, j - N));

    printf("%i\n", Ans.size());
    for(int i = 0; i < Ans.size(); ++ i)
        printf("%i %i\n", Ans[i].first, Ans[i].second);
}

int main()
{
    freopen("harta.in", "r", stdin);
    freopen("harta.out", "w", stdout);

    scanf("%i", &N);

    Source = 0;
    Sink = 2 * N + 1;

    for(int i = 1; i <= N; ++ i)
    {
        scanf("%i %i", &From, &To);
        G[Source].push_back(i);
        G[i].push_back(Source);
        Cap[Source][i] = From;
        G[i + N].push_back(Sink);
        G[Sink].push_back(i + N);
        Cap[i + N][Sink] = To;
    }

    for(int i = 1; i <= N; ++ i)
        for(int j = 1; j <= N; ++ j)
        {
            if(i == j) continue;
            G[i].push_back(j + N);
            G[j + N].push_back(i);
            Cap[i][j + N] = 1;
        }

    MaxFlow();

    return 0;
}