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

using namespace std;

int radacina, destinatie, n, m, dist1, dist2, cost, total, l, r, tati[202], flux[202][202], viz[202];
vector<vector<int>>adiacenta;

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

bool bfs()
{
    queue<int> q;

    for(int i =0; i <= 2 * n + 1; ++i)
    {
        tati[i] = -1;
        viz[i] = 0;
    }
    viz[0] = 1;
    q.push(0);

    while(!q.empty())
    {
        int aux = q.front();
        q.pop();
        for(auto element : adiacenta[aux])
        {
            if(viz[element] == 0 && flux[aux][element] > 0)
            {
                tati[element] = aux;
                q.push(element);
                viz[element] = 1;
                if(element == 2 * n + 1) return 1;
            }
        }
    }
    return 0;
}

int main()
{
    f>>n;
    adiacenta.resize(2 * n + 2);

    for(int i = 1; i <= n; ++i)
    {
        f>>dist1>>dist2;
        flux[0][i] = dist1;
        flux[n + i][2 * n + 1] = dist2;

        adiacenta[0].push_back(i);
        adiacenta[i].push_back(0);
        adiacenta[n + i].push_back(2 * n + 1);
        adiacenta[2 * n + 1].push_back(n + 1);

        for(int j = n + 1; j <= 2 * n; ++j)
        {
            if(j - i == n)
                continue;
            adiacenta[i].push_back(j);
            adiacenta[j].push_back(i);
            flux[i][j] = 1;
        }
    }

    while(bfs())
    {
        for(auto element : adiacenta[n])
        {
            if(!viz[element]) continue;

            int minim = INT_MAX;
            int aux = 2 * n + 1;

            while(aux != 0)
            {
                minim = min(minim, flux[tati[aux]][aux]);
                aux = tati[aux];
            }

            aux = 2 * n + 1;

            while(aux != 0)
            {
                flux[tati[aux]][aux] -= minim;
                flux[aux][tati[aux]] += minim;
                aux = tati[aux];
            }

            total += minim;

        }
    }
    g<<total<<"\n";
    for(int i = 1; i <= n; ++i)
        for(int j = n + 1; j <= 2 * n + 1; ++j)
            if(flux[j][i] == 1)
            {
                g<<i<<" "<<j - n<<"\n";
                continue;
            }
}