#include<iostream>
#include<math.h>
#include<iomanip>
#include<string>
#include<algorithm>
#include<map>
#include<stack>
#include<deque>
#include<cstdio>
#include<queue>
#include<fstream>
using namespace std;
int main() {
 ifstream cin("hobby.in");
 ofstream cout("hobby.out");
 int n,i,j,m,z,x;
 pair< int,pair<int,int> >a[1005];
 cin>>n;
 for(i=0;i<n;i++) {
  cin>>a[i].first>>a[i].second.first;
  a[i].second.second=i+1;
 }
 sort(a+0,a+n);
 cout<<n/2<<endl;
 for(i=1;i<n;i+=2) {
  cout<<a[i-1].second.second<<" "<<a[i].second.second<<endl;
 }
 return 0;
}