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.util.ArrayList;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {


        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        ArrayList<Node> nodes = new ArrayList<>();

        ArrayList<edge> notUsed = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            nodes.add(new Node(i));
        }

        DSU dsu = new DSU();
        for (int i = 0; i < n-1; i++) {
            int a = scanner.nextInt();
            int b = scanner.nextInt();
            a--;b--;
            if (dsu.findSet(nodes.get(a)) != dsu.findSet(nodes.get(b))){
                dsu.Union(nodes.get(a),nodes.get(b));
            }
            else {
                notUsed.add(new edge(nodes.get(a),nodes.get(b)));
            }
        }

        Node v = dsu.findSet(nodes.get(0));
        int number = v.number;
        int numberToujours = number;
        ArrayList<Integer> saving = new ArrayList<>();
        int j = 0;

        for (int i = 1; i < n; i++) {
            Node w = dsu.findSet(nodes.get(i));
            if (w.number != number){
                j++;
                dsu.Union(v,w);
                saving.add(w.number);
                number = dsu.findSet(v).number;
            }
        }


        System.out.println(j);
        for (int i = 0; i < j; i++) {
            System.out.println(notUsed.get(i).start.number+" "+notUsed.get(i).end.number+" "+numberToujours+" "+saving.get(i));
        }

    }
}


class Node{

    Node parent;
    int number;

    public Node(int number){
        this.number = number+1;
        this.parent = this;
    }
}

class edge{
    Node start,end;

    public edge(Node v,Node w){
        this.start = v;
        this.end = w;
    }
}

class DSU{

    Node findSet(Node v){
        if (v.parent == v)
            return v;
        else
            return v.parent=findSet(v.parent);
    }


    void Union(Node v,Node w){
        v = findSet(v);
        w = findSet(w);
        if (v.number > w.number){
            w.parent = v;
        }else {
            v.parent = w;
        }
    }
}