#include <stdio.h>
#include <vector>
#include <queue>
#include <stack>
#include <list>
#include <set>
#include <algorithm>
#include <utility>
#include <string>
#include <functional>
#include <sstream>
#include <fstream>
using namespace std;
#define FOR(i,a,b) for (typeof a i=a;i<=b;++i)
#define fori(it,v) for (typeof ((v).begin()) it=(v).begin();it!=(v).end();it++)
#define pb push_back
#define mp make_pair
#define fs first
#define ss second
#define all(c) c.begin(),c.end()
#define pf push_front
#define popb pop_back
#define popf pop_front
typedef pair<int,int> ii;
int dest;
vector<int> a[300];
int viz[300];
int cap[300][300];
int pred[300];
vector<ii> rez;
int flux()
{
	int y,z=1;
	queue<int> q;
	memset(viz,0,sizeof(viz));
	q.push(0);
	//printf("WTF %d\n",cap[1][6]);
	pred[dest]=0;
	while (!q.empty()&&z)
	{
		int nod=q.front();
		q.pop();
		vector<int>::iterator it;
		fori(it,a[nod])
		{
			//printf("%d %d\n",nod,*it);
			if (cap[nod][*it]>0&&!viz[*it])
			{
	//			printf("Bagat (%d %d)\n",nod,*it);
				pred[*it]=nod;
				q.push(*it);
				viz[*it]=1;
		//		printf("%d %d\n",nod,*it);
				if (*it==dest)
				{
					z=0;
					break;
				}
			}
	//		else	printf("Respins(%d %d)\n",nod,*it);
		}
		viz[nod]=0;
	}	
	if (!pred[dest])
		return 0;
	y=dest;
	//printf("Drum : %d, ",dest);
	while (y)
	{
		cap[pred[y]][y]--;
		cap[y][pred[y]]++;
		y=pred[y];
	//	printf("%d ",y);
	}
//	printf("\n");
	//scanf("%d",&z);
	return 1;
}

int main()
{
	FILE *in,*out;
	int n,i,j,nr,x,y;
	in=fopen("harta.in","r");
	out=fopen("harta.out","w");
	fscanf(in,"%d",&n);
	dest=2*n+1;
	FOR(i,1,n)
	{
		fscanf(in,"%d%d",&x,&y);
		a[0].pb(i);
		a[i].pb(0);
		cap[0][i]=x;
		a[dest].pb(i+n);
		a[i+n].pb(dest);
		cap[i+n][dest]=y;
	}
	FOR(i,1,n)
		FOR(j,1,n)
		if (i!=j)
		{
			a[i].pb(j+n);
			a[j+n].pb(i);
			cap[i][j+n]=1;
		}
	while (flux());
	nr=0;
	FOR(i,1,n)
		FOR(j,1,n)
			if (i!=j&&!cap[i][j+n])
			{
				nr++;
				rez.pb(mp(i,j));
			}
	fprintf(out,"%d\n",nr);
	vector<pair<int,int> >::iterator it;
	fori(it,rez)
	{
		fprintf(out,"%d %d\n",it->fs,it->ss);
	}
	fclose(in);
	fclose(out);
	return 0;
}