#include <fstream>
using namespace std;

ifstream cin("harta.in");
ofstream cout("harta.out");

int a[101];
int b[101];
int cnt[101];
int aux[101][101];
int main()
{
    int n,m=0,i,j,k,aux1;
    cin>>n;
    for(i=1;i<=n;i++){
        cin>>a[i]>>b[i];
        m+=a[i];
    }
    cout<<m<<"\n";
    for(i=1;i<=n;i++){
        for(j=0;j<=100;j++) cnt[j]=0;
        for(j=1;j<=n;j++){
            if(b[j] && i!=j){
                cnt[b[j]]++;
                aux[b[j]][cnt[b[j]]]=j;
            }
        }
        for(j=n;j && a[i];j--){
            for(k=cnt[j];k && a[i];k--){
                aux1=aux[j][k];a[i]--;b[aux1]--;
                cout<<i<<" "<<aux1<<"\n";
            }
        }
    }
    return 0;
}