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

using namespace std;

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

vector<int> in, out;
vector<vector<int> > G(1001);
int mat[1001][1001], n, grad_in, grad_out, total = 0, tata[1001];
bool viz[1001];

bool bfs()
{
   for(int i = 0; i <= 2*n+1; i++)
   {
       viz[i] = 0;
       tata[i] = 0;
   }
    queue<int> coada;
    coada.push(0);
    while(!coada.empty())
    {
        int nod = coada.front();
        coada.pop();
        for(int i = 0; i < G[nod].size(); i++)
        {
            int vecin = G[nod][i];

            if(!viz[vecin] && mat[nod][vecin] > 0)
            {
                viz[vecin] = 1;
                tata[vecin] = nod;
                coada.push(vecin);
            }
        }
    }
    return viz[2*n+1];
}
int main()
{
    f >> n;
    for(int i = 1; i <= n; i++)
    {
        f >> grad_in >> grad_out;
        mat[0][i] = grad_in;
        mat[n+i][2*n+1] = grad_out;

        G[0].push_back(i);
        G[n+i].push_back(2*n+1);
        G[i].push_back(0);
        G[2*n+1].push_back(n+i);
        for(int j = n + 1; j <= 2*n; j++)
        {
            if(i == j - n)
                continue;
            else
            {
                mat[i][j] = 1;
                G[i].push_back(j);
                G[j].push_back(i);
            }
        }
    }
    int flux = 0;
    while(bfs())
    {
        int nod = 2 * n + 1;
        int minn = INT_MAX;
        while(nod != 0)
        {
            minn = min(minn, mat[tata[nod]][nod]);
            nod = tata[nod];
        }
        nod = 2*n+1;
        while(nod != 0)
        {
            mat[nod][tata[nod]] += minn;
            mat[tata[nod]][nod] -= minn;
            nod = tata[nod];
        }
        flux += minn;
    }
    g << flux << '\n';
    for(int i = 1; i <= n; i++)
    {
        for(int j = 0; j < G[i].size(); j++)
        {
            if(G[i][j] && G[i][j] != 2*n+1 && mat[i][G[i][j]] == 0)
                g << i << ' ' << G[i][j] - n << '\n';
        }
    }
    return 0;
}