#ifdef LOCAL
#include <local.hpp>
#else
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2")
#include <bits/stdc++.h>
#define debug(...) ((void)0)
#define postprocess(...) ((void)0)
#endif

#include <atcoder/maxflow.hpp>

using namespace std;
using ll = long long;
using ld = long double;

void solve([[maybe_unused]] int test) {
    int N;
    cin >> N;

    vector<tuple<int, int, int>> points;
    for (int i = 1; i <= N; i++) {
        int x, y;
        cin >> x >> y;
        points.push_back({x, y, i});
    }
    sort(points.begin(), points.end());

    int K = N / 2;
    cout << K << endl;

    for (int i = 0; i < K; i++) {
        int z, w;
        tie(ignore, ignore, z) = points[2 * i];
        tie(ignore, ignore, w) = points[2 * i + 1];
        cout << z << ' ' << w << endl;
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    for (int i = 1; i <= t; i++) {
        solve(i);
    }

    postprocess();
}