#include <bits/stdc++.h>
#define maxN 204
#define inf 1000000000
using namespace std;

FILE *fin = freopen("harta.in", "r", stdin);
FILE *fout = freopen("harta.out", "w", stdout);

/* ================== */
int n, m, N;
/* ================== */
int S, D, r[maxN][maxN], deUnde[maxN];
queue < int > Q;
/* ================== */
int ans;

void BFS(int S, int D)
{
    int i;
    memset(deUnde, 0, sizeof(deUnde));
    Q.push(S);
    deUnde[S] = S;
    deUnde[D] = D;
    while (!Q.empty())
    {
        int x = Q.front();
        Q.pop();
        for (i = 1; i <= N; ++ i)
            if (r[x][i] > 0 && deUnde[i] == 0)
            {
                deUnde[i] = x;
                Q.push(i);
            }
    }
}


void maxFlow()
{
    ans = 0;
    bool continua = true;
    while (continua)
    {
        BFS(S, D);
        continua = false;
        for(int u = 1; u <= N; u++)
            if (r[u][D] > 0 && deUnde[u] != 0 && u != D)
            {
                continua = true;
                deUnde[D] = u;
                int nod = D;
                int cap = inf;
                while (nod != S)
                {
                    cap = min(cap, r[deUnde[nod]][nod]);
                    nod = deUnde[nod];
                }
                nod = D;
                ans += cap;
                while (nod != S)
                {
                    r[deUnde[nod]][nod] -= cap;
                    r[nod][deUnde[nod]] += cap;
                    nod = deUnde[nod];
                }
            }
    }
}

int main()
{
    scanf("%d", &n);
    S = 1;
    D = N = 2 * n + 2;
    for (int i = 1; i <= n; ++ i)
    {
        int gIn, gOut;
        scanf("%d %d", &gIn, &gOut);
        m += gIn + gOut;
        r[S][i * 2 + 1] = gIn;
        r[i * 2][D] = gOut;
    }

    for (int i = 1; i <= n; ++ i)
        for (int j = i + 1; j <= n; ++ j)
            r[i * 2 + 1][j * 2] = r[j * 2 + 1][i * 2] = 1;
    maxFlow();

    printf("%d\n", m / 2);
    for (int i = 1; i <= n; ++ i)
        for (int j = 1; j <= n; ++ j)
            if (i != j && !r[i * 2 + 1][j * 2])
                printf("%d %d\n", i, j);

    return 0;
}