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