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 
/**
 * Alexandra Mikhaylova mikhaylova@yandex-team.ru
 */

import java.util.Scanner;

public class X {
    private int cityCount;
    private int roadCount;

    private DisjointSetsSystem disjointSetsSystem;

    private int[][] extraRoads;
    private int extraRoadsCount;

    private int[][] necessaryRoads;

    public X() {
        Scanner scanner = new Scanner(System.in);
        cityCount = scanner.nextInt();
        roadCount = cityCount - 1;

        disjointSetsSystem = new DisjointSetsSystem(cityCount);

        extraRoads = new int[2][roadCount];
        necessaryRoads = new int[2][roadCount];

        int j = 0;
        for (int i = 0; i < roadCount; i ++) {
            int v1 = scanner.nextInt() - 1;
            int v2 = scanner.nextInt() - 1;
            if (disjointSetsSystem.find(v1) != disjointSetsSystem.find(v2)) {
                disjointSetsSystem.union(v1, v2);
            } else {
                extraRoads[0][j] = v1;
                extraRoads[1][j] = v2;
                j ++;
            }
        }
        extraRoadsCount = j;
    }

    private class DisjointSetsSystem {
        int[] parents;
        int[] rank;

        public DisjointSetsSystem(int size) {
            parents = new int[size];
            for (int i = 0; i < size; i ++) {
                parents[i] = i;
            }
            rank = new int[size];
        }

        public int find(int x) {
            if (parents[x] == x) {
                return x;
            } else {
                return find(parents[x]);
            }
        }

        public void union(int a, int b) {
            a = find(a);
            b = find(b);
            if (a == b) {
                return;
            }
            if (rank[a] < rank[b]) {
                parents[a] = b;
            } else {
                parents[b] = a;
                if (rank[a] == rank[b]) {
                    ++rank[a];
                }
            }
        }
    }

    public void findNecessaryRoads() {
        int l = 0;
        for (int j = 0; j < disjointSetsSystem.parents.length; j ++) {
            for (int k = 0; k < disjointSetsSystem.parents.length; k ++) {
                if (disjointSetsSystem.find(j) != disjointSetsSystem.find(k)) {
                    disjointSetsSystem.union(j, k);
                    necessaryRoads[0][l] = j;
                    necessaryRoads[1][l] = k;
                    l ++;
                    if (l == extraRoadsCount) {
                        return;
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        X road = new X();
        road.findNecessaryRoads();
        System.out.println(road.extraRoadsCount);
        StringBuilder stringBuilder;
        for (int i = 0; i < road.extraRoadsCount; i ++) {
            stringBuilder = new StringBuilder();
            stringBuilder.append(road.extraRoads[0][i] + 1).
                    append(" ").
                    append(road.extraRoads[1][i] + 1).
                    append(" ").
                    append(road.necessaryRoads[0][i] + 1).
                    append(" ").
                    append(road.necessaryRoads[1][i] + 1);
            System.out.println(stringBuilder.toString());
        }
    }
}


