#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001

int ind[8005][8005];

int main(){
	int N;
	cin>>N;
	rep(i,N){
		int x,y;cin>>x>>y;
		ind[x][y] = i+1;
	}
	
	vector<int> a,b;
	rep(i,8005){
		rep(j,8005){
			if(ind[i][j]==0)continue;
			if(a.size()==b.size())a.push_back(ind[i][j]);
			else b.push_back(ind[i][j]);
		}
	}
	if(a.size()>b.size())a.pop_back();
	cout<<a.size()<<endl;
	rep(i,a.size()){
		cout<<a[i]<<' '<<b[i]<<endl;
	}
	
	return 0;
}