#include <bits/stdc++.h>

using namespace std;
void fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
}
int main() {
    fast_io();
    int n;
    cin >> n;
    vector<pair<int, int>> xy(n);
    for (int i = 0; i < n; i++) {
        cin >> xy[i].first >> xy[i].second;
    }
    vector<int> idx(n);
    iota(idx.begin(), idx.end(), 0);
    sort(idx.begin(), idx.end(), [&](int i, int j) { return xy[i] < xy[j]; });
    cout << n / 2 << "\n";
    for (int i = 0; i < n - 1; i += 2) {
        cout << idx[i] + 1 << " " << idx[i + 1] + 1 << "\n";
    }
}