#include <iostream>
#include <vector>
#include <fstream>
#include <queue>
#include <algorithm>
using namespace std;
ifstream fin("harta.in");
ofstream fout("harta.out");

int c[1024][1024]; //capacitate intre doua noduri
int f[1024][1024]; //capacitate consumata
int tata[1024]; //tatal fiecarui nod
vector<int> la[1024];
//int coada[1024];
vector<int> vizitat;
int nod,n,m,x,y,z,i,j,minn,flow,sursa,destinatie;

int bfs()
{
    vizitat.assign(n+n+2, 0);
    queue<int> coada;
	coada.push(0);
	//vizitat[sursa]=1;
	while(!coada.empty())
    {
        nod=coada.front();
        coada.pop();
        vizitat[nod]=1;
        if(nod == n+n+1) break; //opresc parcurgerea daca ajung la destinatie
        for(int j=0;j<la[nod].size();j++)
        {
            if(vizitat[la[nod][j]]==0 && c[nod][la[nod][j]]!=f[nod][la[nod][j]]) //daca nu e vizitat si daca drumul nu a fost consumat
            {
                coada.push(la[nod][j]);
                vizitat[la[nod][j]]=1;
                tata[la[nod][j]]=nod; //retin tatal nodului
            }
        }
    }
    return vizitat[n];//ne zice daca a ajuns la final drumul -> nu s a gasit fluxul maxim deci continuam
}

int main()
{
    fin>>n;
    sursa=0;
    destinatie=n+n+1;
    ///dublez nodurile
    for(i =1;i<=n;i++)
    {
  		fin>>x>>y;
  		la[sursa].push_back(i);
  		la[i].push_back(sursa);
  		la[destinatie].push_back(n+i);
  		la[n+i].push_back(destinatie);
  		c[0][i] =x;
        c[i+n][n+n+1]=y;
  	}
  	for(i =1;i<=n;i++)
    {
        for(j=1+n;j<=n+n;j++)
        {
            if(i!=j-n) //intre nod si dublura lui nu e muchie
            {
                c[i][j]=1;//intre oricare nod si alta dublura am muchie
                la[i].push_back(j);
                la[j].push_back(i);
            }
        }
    }
  	flow=0;
  	while(bfs())
    {
        for(i=0;i<la[n+n+1].size();i++)
        {
            nod = la[n+n+1][i];
			if (f[nod][n+n+1] == c[nod][n+n+1] || !vizitat[nod]) continue;
			tata[n+n+1] = nod;
			int minn=9999999;
			for (j =n+n+1; j>0; j = tata[j])
            {
  				minn = min(minn, c[tata[j]][j] - f[tata[j]][j]); //calculez capacitatea minima a drumului (capacitate totala-consumata)
  			}
			if (minn == 0) continue;
			for (j =n+n+1; j>0; j = tata[j]) //modific capacitatile drumurilor+ maresc flowul rezidual
            {
  				f[tata[j]][j] += minn;
  				f[j][tata[j]] -= minn;
  			}
  			flow=flow+minn;
        }
    }
    fout<<flow<<'\n';
    for (i = 1; i <= n; ++i){
        for (j=0;j<la[i].size();j++)
        {
            if(la[i][j]!=0 && f[i][la[i][j]]!=0)
                fout<<i<<' '<<la[i][j]-n<<'\n';
        }
    }
    return 0;
}