#include <stdio.h>
#include <vector>

using namespace std;

int N, source, sink;
int F[250][250], C[250][250], dad[250], vis[250];
vector <int> G[250];

bool BFS() {
    int i;
    for (i = 0; i <= sink; i ++) {
        dad[i] = 0; vis[i] = 0;
    }

    int p = 1, u = 0;
    int Q[250];
    Q[++ u] = 0; dad[0] = -1; vis[0] = 1;

    while (p <= u) {
        int nod = Q[p];
        vector <int> :: iterator it;
        for (it = G[nod].begin(); it != G[nod].end(); it ++)
            if (F[nod][*it] < C[nod][*it] && vis[*it] == 0) {
                dad[*it] = nod;
                vis[*it] = 1;
                Q[++ u] = *it;
            }
        p ++;
    }

    return dad[sink];
}

void augment() {
    int nod = sink;
    while (nod != source) {
        F[nod][dad[nod]] --;
        F[dad[nod]][nod] ++;
        nod = dad[nod];
    }
}

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

    scanf("%d", &N);
    source = 0; sink = 2 * N + 1;

    int i;
    for (i = 1; i <= N; i ++) {
        int in, out;
        scanf("%d%d", &out, &in);

        G[source].push_back(i); G[i + N].push_back(sink);
        C[source][i] = out; C[i + N][sink] = in;
    }

    int j;
    for (i = 1; i <= N; i ++)
        for (j = N + 1; j <= 2 * N; j ++)
            if (i != j - N) {
                G[i].push_back(j); G[j].push_back(i);
                C[i][j] = 1;
            }

    int sol = 0;
    while (BFS()) {
        sol ++; augment();
    }

    printf("%d\n", sol);
    for (i = 1; i <= N; i ++)
        for (j = N + 1; j <= 2 * N; j ++)
            if (F[i][j] == 1)
                printf("%d %d\n", i, j - N);

    return 0;
}