#include<iostream>
#include<set>
#include<algorithm>
#include<vector>
#include<string>
#include<set>
#include<map>
#include<numeric>
#include<queue>
#include<cmath>
using namespace std;
typedef long long ll;
const ll INF=1LL<<60;
typedef pair<int,int> P;
typedef pair<int,P> PP;
const ll MOD=998244353;
const double PI=acos(-1);

int main(){
    int N;
    cin>>N;
    

    map<int,vector<int>> mp;

    map<P,int> dogid;

    for(int i=0;i<N;i++){
        int x,y;
        cin>>x>>y;
        mp[x].push_back(y);
        dogid[make_pair(x,y)]=i;

    }
    for(auto &[x,vec]:mp){
        sort(vec.begin(),vec.end());
    }

    vector<pair<int,int>> ans;

    int k=0;

    int offset=0;
    
    pair<int,int> empty=make_pair(-1,-1);
    pair<int,int> last=empty;
    
    /*   
    int empty=-1;
    int last=empty;
    */
    
    for(auto& [x,vec]:mp){

     
        if(offset>0){
            ans.emplace_back(dogid[last],dogid[make_pair(x,vec.back())]);
            vec.pop_back();
            offset=0;
            last=empty;
        }
        
        int k=0;
        if(k+1<vec.size()){
        for(k=0;k<vec.size();k+=2){
            ans.emplace_back(dogid[make_pair(x,vec[k])],dogid[make_pair(x,vec[k+1])]);
        }
        }

        if(k!=vec.size()){
            last=make_pair(x,vec.back());
            offset=1;
        }


        /*
        if(c%2==0){
            k+=c/2;
            if(offset>0){
                ans.emplace_back(dogid[last],dogid[make_pair(x,vec.back())]);
            }
            offset=0;

            for(int k=0;k<vec.size();k+=2){
                ans.emplace_back(dogid[make_pair(x,vec[k])],dogid[make_pair(x,vec[k+1])]);
            }


        }else{
            offset=1;//つぎに繰り越し

            k+=(c-1)/2;


        }
        */
    }

    cout<<ans.size()<<endl;
    for(auto [z,w]:ans){
        cout<<z+1<<' '<<w+1<<endl;
    }
}