#include <bits/stdc++.h>
// #include <atcoder/modint>
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define INF 2000000000000000000
#define ll long long
#define ull unsigned long long
#define ld long double
#define pll pair<ll, ll>
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
const double PI = 3.141592653589793238462643383279;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  map<ll, vector<ll>> X;
  map<pll, ll> index;
  ll N;
  cin >> N;
  for (ll i = 0; i < N; ++i) {
    ll x, y;
    cin >> x >> y;
    index[{x, y}] = i;
    X[x].push_back(y);
  }
  pll before = {-1, -1};
  vector<pll> ans;
  for (const auto& [key, temp] : X){
    vector<ll> value = temp;
    sort(rng(value));
    for (ll i = 0; i < (ll)value.size(); ++i) {
      ll index1 = index[{key, value.at(i)}];
      if (before.first != -1) {
        ans.push_back({index[before] + 1, index1 + 1});
        before = {-1, -1};
      }
      else {
        before = {key, value.at(i)};
      }
    }
  }
  cout << ans.size() << "\n";
  for (ll i = 0; i < ans.size(); ++i) {
    cout << ans.at(i).first << ' ' << ans.at(i).second << "\n";
  }
}