#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>

using namespace std;

int main(){
	int n, a, b;
	cin >> n;
	for(int i = 0; i < n - 1; ++i){
		cin >> a >> b;
	}
	vector< pair<pair<int, int>, int > > p;
	for(int i = 0; i < n; ++i){
		cin >> a >> b;
		p.push_back(make_pair(make_pair(a, b), i));
	}
	cout << p.size() << endl;
	sort(p.begin(), p.end());
	for(int i = 0; i < n - 1; ++i){
		cout << p.at(i).second << " " << p.at(i + 1).second << endl;
		if(i == n - 2)
			cout << endl;
		else
			cout << " ";
	}
	return 0;
}