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.*;

public class TaskX {
    private int path, city, unnecessaryNumber;
    private ItemSample itemSample;
    private int[][] unnecessaryItems, necessaryItems;

    public TaskX() {
//        get input data
        Scanner scan = new Scanner(System.in);
        city = scan.nextInt();
        path = city - 1;
        itemSample = new ItemSample(city);
        unnecessaryItems = new int[2][path];
        necessaryItems = new int[2][path];
        int j = 0;
        for (int i = 0; i < path; i++) {
            int firstNode = scan.nextInt() - 1;
            int secondNode = scan.nextInt() - 1;
//          join components or add this item to unnecessary items if nodes don't pertain to joined component
            if (itemSample.root(firstNode) != itemSample.root(secondNode)) {
                itemSample.join(firstNode, secondNode);
            } else {
                unnecessaryItems[0][j] = firstNode;
                unnecessaryItems[1][j] = secondNode;
                j++;
            }
        }
        unnecessaryNumber = j;
    }

    public void findNeededEdges() {
        int l = 0;
        int j = 0;
        //for (int j = 0; j < itemSample.item.length; j++) {
            for (int k = 0; k < itemSample.item.length; k++) {
//             join nodes if they dont't pertain to joined component
                if (itemSample.root(j) != itemSample.root(k)) {
                    itemSample.join(j, k);
                    necessaryItems[0][l] = j;
                    necessaryItems[1][l] = k;
                    l++;
                    if(l == unnecessaryNumber){
                        return;
                    }
                }
            
        }
    }

    private class ItemSample {
        int[] item;
        int[] rank;

        public ItemSample(int size) {
            item = new int[size];
            for (int i = 0; i < size; i++) {
                item[i] = i;
            }
            rank = new int[size];
        }
        public void join(int m, int n) {
            m = root(m);
            n = root(n);
            if (m == n)
                return;
            if (rank[m] < rank[n]) {
                item[m] = n;
            } else {
                item[n] = m;
                if (rank[m] == rank[n])
                    ++rank[m];
            }
        }
        public int root(int x) {
            while (x != item[x])
                x = item[x];
            return x;
        }
    }

    public static void main(String[] args) {
        TaskX taskX = new TaskX();
        taskX.findNeededEdges();
        System.out.println(taskX.unnecessaryNumber);
        StringBuilder output;
        for (int i = 0; i < taskX.unnecessaryNumber; i++) {
            output = new StringBuilder();
            output.append(taskX.unnecessaryItems[0][i] + 1).
                    append(" ").
                    append(taskX.unnecessaryItems[1][i] + 1).
                    append(" ").
                    append(taskX.necessaryItems[0][i] + 1).
                    append(" ").
                    append(taskX.necessaryItems[1][i] + 1);
            System.out.println(output.toString());

        }

    }
}