#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  vector<int> x(N), y(N);
  for (int i = 0; i < N; i++){
    cin >> x[i] >> y[i];
  }
  vector<tuple<int, int, int>> T(N);
  for (int i = 0; i < N; i++){
    T[i] = make_tuple(x[i], y[i], i);
  }
  sort(T.begin(), T.end());
  cout << N / 2 << endl;
  for (int i = 0; i < N / 2; i++){
    cout << get<2>(T[i * 2]) + 1 << ' ' << get<2>(T[i * 2 + 1]) + 1 << endl;
  }
}