typedef long long ll;
typedef long double ld;
#include <bits/stdc++.h>
using namespace std;
#define int long long

signed main(){
    ll n;
    std::cin >> n;
    vector<tuple<ll,ll,ll>> xy(n);
    vector<ll> ans;
    for (int i = 0; i < n; i++) {
        ll x,y;
        std::cin >> x>>y;
        xy[i] = {x,y,i};
    }
    sort(xy.begin(),xy.end());

    std::cout << xy.size()/2 << std::endl;
    for (int i = 0; i < xy.size()/2; i++) {
        auto [t1,t2,t3] = xy[2*i];
        auto [t12,t22,t32] = xy[2*i+1];
        std::cout << t3+1<<" "<<t32+1 << std::endl;
    }
}