#include <fstream>
#include <queue>
#include <vector>
#define inf 1e9
using namespace std;
ifstream cin("harta.in");
ofstream cout("harta.out");
int c[205][205],flow[205][205],dist[205],m,n;
vector<int>v[205];
queue<int>q;
bool bfs(int s,int d){
    for(int i=0;i<=2*n+1;i++)
        dist[i]=inf;
    q.push(s);
    dist[s]=1;
    while(!q.empty()){
        int nod=q.front();
        q.pop();
        for(auto i:v[nod])
            if(dist[i]>dist[nod]+1&&flow[nod][i]<c[nod][i]){
                dist[i]=dist[nod]+1;
                q.push(i);
            }
    }
    return dist[d]!=inf;
}
int dinic(int nod,int d,int val){
    if(val==0)
        return 0;
    if(nod==d)
        return val;
    int total=0;
    for(auto i:v[nod])
        if(dist[i]==dist[nod]+1){
            int add=dinic(i,d,min(c[nod][i]-flow[nod][i],val-total));
            flow[nod][i]+=add;
            flow[i][nod]-=add;
            total+=add;
        }
    return total;
}
signed main()
{
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>c[0][i]>>c[i+n][2*n+1];
        v[0].push_back(i);
        v[i].push_back(0);
        v[i+n].push_back(2*n+1);
        v[2*n+1].push_back(i+n);
    }
    for(int i=1;i<=n;i++)
        for(int j=1;j<=n;j++)
            if(i!=j){
                v[i].push_back(j+n);
                v[j+n].push_back(i);
                c[i][j+n]=1;
            }
    while(bfs(0,2*n+1))
        m+=dinic(0,2*n+1,inf);
    cout<<m<<'\n';
    for(int i=1;i<=n;i++)
        for(int j=1;j<=n;j++)
            if(flow[i][j+n]==1)
                cout<<i<<" "<<j<<'\n';
    return 0;
}