#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin("harta.in");
ofstream cout("harta.out");
int n,total;
struct elem{
    int val,nod;
}v[101],aux[101];/// in v -> out in aux -> in
bool cmp(const elem& a,const elem& b){
    if(a.val==b.val)
        return a.nod>b.nod;
    return a.val>b.val;
}
signed main()
{
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>v[i].val>>aux[i].val;
        total+=v[i].val;
        v[i].nod=aux[i].nod=i;
    }
    cout<<total<<'\n';
    sort(v+1,v+n+1,cmp);
    for(int i=1;i<=n;i++){
        sort(aux+1,aux+n+1,cmp);
        for(int j=1;v[i].val&&j<=n;j++)
            if(v[i].nod!=aux[j].nod&&aux[j].val){
                cout<<v[i].nod<<" "<<aux[j].nod<<'\n';
                v[i].val--;
                aux[j].val--;
            }
    }
    return 0;
}