#include<bits/stdc++.h>
using namespace std;
using ll=long long;
using T=tuple<ll,ll,ll>;

int main(){
  ll n;
  cin>>n;
  vector<T> v(n);
  for(ll i=0;i<n;i++){
    ll x,y;
    cin>>x>>y;
    v[i]=T(x,y,i+1);
  }
  sort(v.begin(),v.end());
  cout<<n/2<<endl;
  for(ll i=0;i<n;i+=2){
    if(i+1<n){
      cout<<get<2>(v[i])<<" "<<get<2>(v[i+1])<<endl;
    }
  }
}