#include <bits/stdc++.h>

using namespace std;

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

vector<vector<int>> adj;
int cap[2005][2005], tata[2005], n, m, s, t;

bool bfs()
{
    queue <int> q;
    vector <bool> vizitat(n * 2 + 2, false);

    q.push(s);
    vizitat[s] = true;
    tata[s] = -1;

    while(!q.empty())
    {
        int nod = q.front();
        q.pop();
        for(auto vecin : adj[nod])
        {
            if(!vizitat[vecin] && cap[nod][vecin] != 0)
            {
                tata[vecin] = nod;
                if(vecin == t)
                    return true;

                vizitat[vecin] = true;
                q.push(vecin);
            }
        }
    }
    return false;
}

int maxflow()   //Alg. Edmonds Karp
{
    int maxim = 0;
    while(bfs())
    {
        int b, a = t, flow = INT_MAX;
        while(a != s)
        {
            b = tata[a];

            if(cap[b][a] < flow)
                flow = cap[b][a];

            a = b;
        }

        a = t;
        while(a != s)
        {
            b = tata[a];
            cap[b][a] -= flow;
            cap[a][b] += flow;
            a = b;
        }

        maxim += flow;
    }

    return maxim;
}

int main()
{
    int a, b, i, j;
    f >> n;
    t = n * 2 + 1;  //sink
    adj.resize(t+1);

    for(i = 1; i <= n; i++)
    {
        for(j = 1; j <= n; j++)
        {
            if(i != j)
            {
                adj[i].push_back(j+n);
                adj[j+n].push_back(i);
                cap[i][j+n] = 1;
            }
        }
        f>>a>>b;
        adj[s].push_back(i);
        adj[i].push_back(s);
        cap[s][i] = a;
        adj[t].push_back(i+n);
        adj[i+n].push_back(t);
        cap[i+n][t] = b;
    }

    g << maxflow() << endl;

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

    return 0;
}