#include <iostream>
#include <vector>
#include <cstring>
#include <queue>
#include <fstream>
#define INF 0x3f3f3f3f
#define NMAX 203
using namespace std;
queue <int> q;
const int debug = 0;
vector <int> v[NMAX];
int dad[NMAX], flux[NMAX][NMAX], cap[NMAX][NMAX];
void bfs (int start, int stop)
{
    memset(dad, -1,  sizeof(dad));
    q.push(start);
    dad[start] = 0;
    while (!q.empty())
    {
        int k = q.front();
        q.pop();
        if (k != stop)
        {
            for (int i = 0; i < v[k].size(); i++)
                if (dad[v[k][i]] == -1 && cap[k][v[k][i]] > flux[k][v[k][i]])
                {
                    
                    dad[v[k][i]] = k;
                    q.push(v[k][i]);

                }
        }
    }

}
int flow (int source, int dest)
{
    
    int sol = 0;
    while (1)
    {
        
        bfs (source, dest);
        if (debug){
            for (int i = source; i <= dest; i++)
                cout << dad[i] << " ";
            cout << "\n";
        }
        if (dad[dest] == -1)
            break;
       // cout << "here";
        for (int i = 0; i < v[dest].size(); i++)
        {
            int nod = v[dest][i];
            if (dad[nod] != -1 && flux[nod][dest] < cap[nod][dest])
            {
                dad[dest] = nod;
                int capmin = INF;
                int p = dest;
            
                while (p > source)
                {
                    capmin = min (capmin, cap[dad[p]][p] - flux[dad[p]][p]);
                    p = dad[p];
                }

                p = dest;

                while (p > source)
                {
                    flux[dad[p]][p] += capmin;
                    flux[p][dad[p]] -= capmin;
                    p = dad[p];
                }
                sol += capmin;
             }

        }
    }
    return sol;
}   
int n, x, y;
int main ()
{
    ifstream cin ("harta.in");
    ofstream cout ("harta.out");
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> x >> y;
        v[i].push_back(0);
        v[0].push_back(i);
        v[i + n].push_back(2 * n + 1);
        v[2 * n + 1].push_back(i + n);
        cap[0][i] = x;
        cap[i + n][2 * n + 1] = y;
        for (int j = 1; j <= n; j++)
            if (i != j)
            {
                cap[i][j + n] = 1;
                v[i].push_back(j + n);
                v[j + n].push_back(i);
            }
    }
    cout << flow(0, 2 * n + 1) << "\n";
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            if (flux[i][j + n] != 0)
                cout << i << " " << j << "\n";
    return 0;
}