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

using namespace std;

const int Mx = 300;

int n,last,sol;
int parent[Mx];
int flow[Mx][Mx],capacity[Mx][Mx];
bool visited[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(visited,0,sizeof(visited));
    visited[0] = 1;

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

        dq.pop_front();

        if (x == last)
           continue;

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

            if (visited[y] || capacity[x][y] == flow[x][y])
               continue;

            visited[y] = 1;
            dq.push_back(y);
            parent[y] = x;
        }
    }

    return visited[last];
}

int max_flow()
{
    int sol = 0;

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

            if (!visited[last] || capacity[x][last] == flow[x][last])
               continue;

            parent[last] = x;

            for (x = last;x;x = parent[x])
                y = min(y,capacity[parent[x]][x] - flow[parent[x]][x]);

            if (!y)
               continue;

            for (x = last;x;x = parent[x])
            {
                flow[parent[x]][x] += y;
                flow[x][parent[x]] -= y;
            }

            sol += y;
        }
    }

    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);
         sol += a;
         add_edge(0,i,a);
         add_edge(n + i,last,b);
     }

     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 = 0;i <= 2 * n + 1;printf("\n"),i++)
         //for (int j = 0;j <= 2 * n + 1;printf("%d ",flow[i][j]),j++);

     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;
}