#include<stdio.h>
#include<string.h>
#include<vector>
using namespace std;

#define mp make_pair
#define INF 2000000000
#define pb push_back
#define pi pair<int,int>
#define x first
#define y second
#define NMAX 206
#define minim(a,b) (a<b ? a : b)

pi rec[NMAX*NMAX];
char viz[NMAX];
int q[NMAX],pred[NMAX];
int n,m,f[NMAX][NMAX],sol;
int cap[NMAX][NMAX],nr;
vector<int> v[NMAX];

int bfs(int start)
{
    int inc,sf,lim,vec,nod,i;
    memset(viz,0,sizeof(viz));
    memset(pred,-1,sizeof(pred));
    q[1]=start;
    viz[start]=1;
    inc=sf=1;
    while(inc<=sf)
    {
        nod=q[inc];
        lim=v[nod].size();
        for(i=0;i<lim;i++)
            if(!viz[vec=v[nod][i]] && f[nod][vec]<cap[nod][vec])
            {
                q[++sf]=vec;
                viz[vec]=1;
                pred[vec]=nod;
            }
        inc++;
    }
    return viz[2*n+2];
}

int main ()
{
    int i,j,poz,a,b,val;
    freopen("harta.in","r",stdin);
    freopen("harta.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%d%d",&a,&b);
        v[2*n+1].pb(2*i-1);
        cap[2*n+1][2*i-1]=a;
        v[2*i-1].pb(2*n+1);
        
        cap[2*i][2*n+2]=b;
        v[2*i].pb(2*n+2);
        v[2*n+2].pb(2*i);
    }
    for(i=1;i<=n;i++)
        for(j=1;j<=n;j++)
            if(i!=j)
            {
                v[2*i-1].pb(2*j);
                v[2*j].pb(2*i-1);
                cap[2*i-1][2*j]=1;
            }
    while(bfs(2*n+1))
    {
        val=INF;
        for(poz=2*n+2;poz!=2*n+1;poz=pred[poz])
            val=minim(val,cap[pred[poz]][poz]-f[pred[poz]][poz]);
        sol+=val;
        for(poz=2*n+2;poz!=2*n+1;poz=pred[poz])
        {
            f[pred[poz]][poz]+=val;
            f[poz][pred[poz]]-=val;
        }
    }
    for(i=1;i<=n;i++)
        for(j=1;j<=n;j++)
            if(f[2*i-1][2*j]==1)
                rec[++nr]=mp(i,j);
    printf("%d\n",nr);
    for(i=1;i<=nr;i++)
        printf("%d %d\n",rec[i].x,rec[i].y);
    return 0;
}