#include <iostream> #include <vector> #include <algorithm> #include <utility> using namespace std; int main(){ int N; cin >> N; vector<int> x(N); vector<int> y(N); for (int i = 0; i < N; i++){ cin >> x[i] >> y[i]; } vector<pair<pair<int,int>, int>> p(N); for (int i = 0; i < N; i++){ pair<int,int> p_stat = pair<int,int>(x[i], y[i]); p[i] = pair<pair<int,int>, int>(p_stat, i); } sort(p.begin(), p.end()); cout << N / 2 << endl; for (int i = 0; i < N - 1; i += 2){ cout << p[i].second + 1 << " " << p[i + 1].second + 1 << endl; } }