#include <stdio.h>
#include <string.h>
#include <vector>
#include <queue>
#include <algorithm>

using namespace std;

const int Mx = 300;

int n,last;
int parent[Mx];
int flow[Mx][Mx],capacity[Mx][Mx];
bool used[Mx];
vector< int > G[Mx];
deque< int > dq;

void add_edge(int a,int b,int c)
{
    G[a].push_back(b);
    G[b].push_back(a);
    capacity[a][b] += c;
}

bool bfs()
{
    dq.clear();
    dq.push_back(0);
    memset(used,0,sizeof(used));
    used[0] = 1;

    while (!dq.empty())
    {
        int a = dq.front();

        dq.pop_front();

        if (a == last)
           continue;

        for (int i = 0;i < G[a].size();i++)
        {
            int b = G[a][i];

            if (used[b] || capacity[a][b] == flow[a][b])
               continue;

            used[b] = 1;
            dq.push_back(b);
            parent[b] = a;
        }
    }

    return used[last];
}

int max_flow()
{
    int sol = 0;

    while (bfs())
    {
        for (int i = 0;i < G[last].size();i++)
        {
            int a = G[last][i],b = 1;

            if (!used[a] || capacity[a][last] == flow[a][last])
               continue;

            parent[last] = a;

            for (a = last;a != 0;a = parent[a])
                b = min(b,capacity[parent[a]][a] - flow[parent[a]][a]);

            if (!b)
               continue;

            for (a = last;a != 0;a = parent[a])
            {
                flow[parent[a]][a] += b;
                flow[a][parent[a]] -= b;
            }

            sol += b;
        }
    }

    return sol;
}

int main()
{
    freopen("harta.in","r",stdin);
    freopen("harta.out","w",stdout);

     scanf("%d",&n);
     last = 2 * n + 1;

     for (int i = 1;i <= n;i++)
     {
         int a,b;
         scanf("%d %d",&a,&b);
         add_edge(0,i,b);
         add_edge(n + i,last,a);
     }

     for (int i = 1;i <= n;i++)
         for (int j = 1;j <= n;j++)
             if (i != j)
                add_edge(i,n + j,1);

     printf("%d\n",max_flow());

     for (int i = 1;i <= n;i++)
         for (int j = 1;j <= n;j++)
             if (flow[i][n + j] == 1)
                printf("%d %d\n",i,j);

  return 0;
}