#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <atcoder/modint>
using namespace std;
using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;
#define rep(i,n) for(int i=0; i<(int)(n); i++)
const i64 INF = 1001001001001001001;

using Modint = atcoder::static_modint<998244353>;

int main(){
    int N; cin >> N;
    vector<pair<int,int>> A(N); rep(i,N) cin >> A[i].first >> A[i].second;
    vector<int> I(N); rep(i,N) I[i] = i;
    sort(I.begin(), I.end(), [&](int l, int r){ return A[l] < A[r]; });
    cout << (N/2) << '\n';
    rep(i,N/2){
        cout << (I[i*2]+1) << ' ' << (I[i*2+1]+1) << '\n';
    }
    return 0;
}



struct ios_do_not_sync{
    ios_do_not_sync(){
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
    }
} ios_do_not_sync_instance;