#include<bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N;
    cin >> N;
    vector<int>x(N),y(N);
    vector<array<int,3>>tmp;
    for(int i = 0; i < N; i++) {
        cin >> x[i] >> y[i];
        tmp.push_back({x[i],y[i],i+1});
    }
    sort(tmp.begin(),tmp.end());
    cout << N/2 << "\n";
    for(int i = 0; i+1 < N; i += 2) {
        cout << tmp[i][2] << " " << tmp[i+1][2] << "\n";
    }
}