#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using pll=pair<ll,ll>;
using tll=tuple<ll,ll,ll>;
using ld=long double;
const ll INF=(1ll<<60);
#define rep(i,n) for (ll i=0;i<(ll)(n);i++)
#define all(v) v.begin(),v.end()
template<class T> void chmin(T &a,T b){
    if(a>b){
        a=b;
    }
}
template<class T> void chmax(T &a,T b){
    if(a<b){
        a=b;
    }
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll n;
    cin >> n;
    cout << n/2 << '\n';
    vector<vector<pll>> v(8001);
    rep(i,n){
        ll l,r;
        cin >> l >> r;
        v[l].push_back({r,i});
    }
    vector<pll> ans;
    ll cnt=0;
    rep(i,8001){
        if(v[i].empty()) continue;
        sort(all(v[i]));
        if(cnt%2==1) reverse(all(v[i]));
        for(auto &j:v[i]){
            ans.push_back(j);
        }
        cnt++;
    }
    for(ll i=0;i<=n-2;i+=2){
        cout << ans[i].second+1 << ' ' << ans[i+1].second+1 << '\n';
    }
}