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

using namespace std;

vector<int> graf[2001];
int tata[205] = {}, viz[205] = {}, cap[205][205];
int n;

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

bool BFS()
{
    queue<int> q;
    for(int i=1;i<205;i++)
        tata[i]=0, viz[i]=0;

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

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

        if (u == 2*n+1)
            return true;

        for (int i = 0; i < graf[u].size(); i++)
        {
            int v = graf[u][i];
            if (cap[u][v] > 0 && !viz[v])
            {
                tata[v] = u;
                viz[v] = 1;
                q.push(v);
            }
        }
    }
    return false;
}

int EdmondsKarp()
{
    int cmax = 0; //nr total de drumuri
    int cmin, u, v;
    while (BFS())
    {
        for (int i = 0; i < graf[2*n+1].size(); i++)
        {
            u = graf[2*n+1][i];
            if (cap[u][2*n+1] > 0 && viz[u])
            {
                cmin = cap[u][2*n+1];
                while (u)
                {
                    v = tata[u];
                    cmin = min(cmin, cap[v][u]);
                    u = v;
                }
                cmax += cmin;

                u = graf[2*n+1][i];
                cap[u][2*n+1] -= cmin;
                cap[2*n+1][u] += cmin;

                while (tata[u] != -1)
                {
                    v = tata[u];
                    cap[v][u] -= cmin;
                    cap[u][v] += cmin;
                    u = v;
                }
            }
        }
    }
    return cmax;
}

int main()
{
    int x, y;
    f>>n;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            if (i != j)
            {
                graf[i].push_back(j+n);
                graf[j+n].push_back(i);
                cap[i][j+n] = 1;
            }
    for (int i = 1; i <= n; i++)
    {
        f>>x>>y;

        graf[0].push_back(i);
        graf[i].push_back(0);
        cap[0][i] = x;

        graf[2*n+1].push_back(i+n);
        graf[i+n].push_back(2*n+1);
        cap[i+n][2*n+1] = y;
    }

    g<<EdmondsKarp()<<endl;

    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            if (cap[j+n][i] == 1)
                g<<i<<" "<<j<<endl;
    return 0;
}