4
1 4
3 1
3 4
 5
4 1
4 3
5 3
2 4
 21
7 15
13 1
14 3
4 10
2 3
16 18
17 20
16 20
8 4
3 12
2 17
13 11
16 1
13 2
13 5
8 9
6 14
3 17
16 9
13 8
 3
3 1
3 2
 6
5 2
5 3
1 4
3 1
5 6
 60
17 34
46 22
50 44
46 33
41 21
31 33
48 6
38 19
35 60
2 24
49 29
7 53
34 1
19 55
32 1
31 42
27 28
4 53
6 50
21 34
1 10
12 36
54 8
16 13
51 43
45 51
54 20
13 53
34 33
49 33
51 11
59 34
15 5
59 28
30 39
13 30
58 4
34 14
3 9
19 34
4 18
26 48
56 54
3 43
57 25
3 41
35 3
48 44
19 13
54 1
23 31
59 47
5 1
46 40
6 26
20 25
37 5
17 24
20 52
 3
3 2
1 2
 39
6 13
15 39
10 35
31 28
4 21
12 39
3 7
3 13
6 1
5 14
36 28
12 15
18 38
30 29
19 34
36 16
20 22
8 13
38 32
26 39
21 37
1 7
15 27
12 26
8 3
6 14
29 2
25 23
32 21
5 16
32 25
6 8
13 10
23 30
34 37
29 33
28 14
36 5
 10
5 9
8 5
7 6
7 9
3 9
2 1
7 2
3 6
7 1

1
3 4 1 2  0
 3
13 2 1 7 3 17 1 19 13 8 1 21  0
 0
 2
48 44 1 6 6 26 1 12  0
 7
12 15 1 2 1 7 1 9 12 26 1 11 8 3 1 12 6 8 1 17 28 14 1 20 36 5 1 24  2
3 6 1 4 7 1 1 10 

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;

/**
 * Created by first on 8/12/2018.
 */
public class RoadsNotOnlyInBerland_D {
    public static void main(String  [] args) throws IOException {
        RoadsNotOnlyInBerland_D x = new RoadsNotOnlyInBerland_D();
        x.solve();
    }
    public void solve() throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int numOfCities = Integer.parseInt(br.readLine());
        ArrayList<Edge> addedRoads = new ArrayList<>();
        ArrayList<Edge> removedRoads = new ArrayList<>();

        Node [] cities = new Node[numOfCities];

        for(int i = 0; i < numOfCities; i++){
            cities[i] = new Node(i);
        }
        for(int i = 0; i < numOfCities - 1; i++){
            String [] inputs = br.readLine().split(" ");
            int from = Integer.parseInt(inputs[0]) - 1;
            int to = Integer.parseInt(inputs[1]) - 1;
            if(find(cities[from]) != find(cities[to])){
                union(cities[from],cities[to]);
            }else {
                removedRoads.add(new Edge(from, to));
            }
        }

        for(int i = 0; i < numOfCities - 1; i++){
            Node p1 = find(cities[i]);
            Node p2 = find(cities[i+1]);
            if(p1 != p2){
                addedRoads.add(new Edge(p1.index,p2.index));
                union(p1,p2);
            }
        }
        System.out.println(removedRoads.size());
        for(int i = 0; i < removedRoads.size(); i++){
            Edge e1 = removedRoads.get(i);
            Edge e2 = addedRoads.get(i);
            System.out.println((e1.from+1) + " " + (e1.to+1) + " " + (e2.from+1) + " " + (e2.to+1));
        }
    }

    private Node union(Node n1, Node n2){
        Node p1 = find(n1);
        Node p2 = find(n2);
        if(p1.rank < p2.rank){
            p1.parent = p2;
            return p2;
        } else {
            p2.parent = p1;
            if(p1.rank == p2.rank){
                p1.rank++;
            }
            return p1;
        }
    }
    private Node find(Node node){
        if(node.parent != node){
            node.parent = find(node.parent);
        }
        return node.parent;
    }

    private class Node{
        int rank = 0;
        int index;
        Node parent = this;
        public Node(int index){
            this.index = index;
        }
    }
    private class Edge{
        int from,to;
        public Edge(int from, int to){
            this.from = from;
            this.to = to;
        }
    }
}
