#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include "debug.hpp"
#else
#define debug(...) 1
#endif

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    cin >> n;
    vector<pair<int, int>> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i].first >> a[i].second;
    }
    vector<int> idx(n);
    iota(idx.begin(), idx.end(), 0);
    sort(idx.begin(), idx.end(), [&] (int i, int j) {
        return a[i] < a[j];
    });
    cout << n / 2 << '\n';
    for (int i = 0; i < n / 2; i++) {
        cout << idx[2 * i] + 1 << ' ' << idx[2 * i + 1] + 1 << '\n';
    }
}