#include <bits/stdc++.h>
using namespace std;
ifstream f ("harta.in");
ofstream g ("harta.out");
const int nmax=3e2+3;
int n,m,act,ant[nmax],usu[nmax][nmax],p[nmax][nmax],sol,x,y,c;
bool viz[nmax];
vector <int> v[nmax];
vector < pair <int,int> > pwp;
queue <int> q;
bool solve()
{
    memset(viz,0,sizeof(viz));
    q.push(2*n+2);
    viz[2*n+2]=1;
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        if(nod==2*n+1) continue;
        for(int i=0;i<v[nod].size();++i)
        {
            int urm=v[nod][i];
            if(usu[nod][urm]==p[nod][urm]||viz[urm]) continue;
            q.push(urm);
            viz[urm]=1;
            ant[urm]=nod;
        }
    }
    return viz[2*n+1];
}
int main()
{
    ios::sync_with_stdio(false);
    f>>n;
    for(int i=1;i<=n;++i)
    {
        f>>x>>y;
        usu[2*n+2][i]+=x;
        v[2*n+2].push_back(i);
        v[i].push_back(2*n+2);
        usu[i+n][2*n+1]+=y;
        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) continue;
            usu[i][j+n]+=1;
            v[i].push_back(j+n);
            v[j+n].push_back(i);
        }
    }
    while(true)
    {
        int ok=solve();
        for(int i=0;i<v[2*n+1].size();++i)
        {
            int nod=v[2*n+1][i];
            if(p[nod][2*n+1]==usu[nod][2*n+1]||!viz[nod]) continue;
            ant[2*n+1]=nod;
            act=2e9;
            for(nod=2*n+1;nod!=2*n+2;nod=ant[nod]) act=min(act,usu[ant[nod]][nod]-p[ant[nod]][nod]);
            if(!act) continue;
            for(nod=2*n+1;nod!=2*n+2;nod=ant[nod])
            {
                p[ant[nod]][nod]+=act;
                p[nod][ant[nod]]-=act;
            }
            sol+=act;
        }
        if(!ok) break;
    }
    g<<sol<<'\n';
    for(int i=1;i<=n;++i)
    {
        for(int j=1;j<=n;++j)
        {
            if(i==j) continue;
            if(p[i][j+n]) g<<i<<' '<<j<<'\n';
        }
    }
    return 0;
}