#include <iostream>
#include <vector>
#include <queue>
#include <algorithm>
#include <cstring>
#include <fstream>

using namespace std;

ifstream in("harta.in");
ofstream out("harta.out");

int capacity[201][201];
vector<int> l[2001];
int tata[201] = { 0 }, viz[202] = { 0 };
///nr de muchii
int n;

int bfs()
{
    queue<int> q;
    memset(tata, 0, sizeof(tata));
    memset(viz, 0, sizeof(viz));

    q.push(0);
    viz[0] = 1;
    tata[0] = -1;

    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        if (u == 2 * n + 1)
            return 1;
        for (int i = 0; i < l[u].size(); i++)
        {
            int v = l[u][i];
            if (capacity[u][v] > 0 && !viz[v])
            {
                tata[v] = u;
                viz[v] = 1;
                q.push(v);
            }
        }
    }
    return 0;
}

int edmonds_karp()
{
    int maxim = 0, minim, u, v;
    while (bfs())
    {
        for (int i = 0; i < l[2 * n + 1].size(); i++)
        {
            u = l[2 * n + 1][i];
            if (capacity[u][2 * n + 1] > 0 && viz[u])
            {
                minim = capacity[u][2 * n + 1];
                while (tata[u] != -1)
                {
                    v = tata[u];
                    minim = min(minim, capacity[v][u]);
                    u = v;
                }
                maxim += minim;
                u = l[2 * n + 1][i];
                capacity[u][2 * n + 1] -= minim;
                capacity[2 * n + 1][u] += minim;
                while (tata[u] != -1)
                {
                    v = tata[u];
                    capacity[v][u] -= minim;
                    capacity[u][v] += minim;
                    u = v;
                }
            }
        }
    }
    return maxim;
}

int main()
{
    int di, de;
    in >> n;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            if (i != j)
            {
                l[i].push_back(j + n);
                l[j + n].push_back(i);
                capacity[i][j + n] = 1;
            }
    for (int i = 1; i <= n; i++)
    {
        in >> di >> de;
        l[0].push_back(i);
        l[i].push_back(0);
        capacity[0][i] = di;
        l[2 * n + 1].push_back(i + n);
        l[i + n].push_back(2 * n + 1);
        capacity[i + n][2 * n + 1] = de;
    }

    out << edmonds_karp() << "\n";

    for (int i = 1; i <= n; i++)
        for (int j = n + 1; j <= 2 * n; j++)
            if (capacity[j][i] == 1)
                out << i << " " << j - n << "\n";
    return 0;
}