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 DRoads {

    public static void main(String[] args) {
        List<List<Integer>> g = new ArrayList<>();
        Scanner s = new Scanner(System.in);
        int n = s.nextInt();
        List<Integer> comps = new ArrayList<>();
        Set<Edge> edges = new HashSet<>();
        boolean[] visited = new boolean[n];
        for (int i = 0; i < n; i++) {
            g.add(new ArrayList<>());
        }
        for (int i = 0; i < n - 1; i++) {
            int a = s.nextInt() - 1;
            int b = s.nextInt() - 1;
            g.get(a).add(b);
            g.get(b).add(a);
            edges.add(new Edge(a, b));
        }
        for (int i = 0; i < n; i++) {
            if (!visited[i]) {
                Queue<Integer> q = new LinkedList<Integer>();
                q.add(i);
                comps.add(i);
                visited[i] = true;
                while (!q.isEmpty()) {
                    Integer cur = q.poll();
                    for (int e : g.get(cur)) {
                        if (e != cur) {
                            if (!visited[e]) {
                                q.add(e);
                                edges.remove(new Edge(cur, e));
                                edges.remove(new Edge(e, cur));
                                visited[e] = true;
                            }
                        }
                    }
                }
            }
        }
        Queue<Edge> ex = new LinkedList<>(edges);
        if (comps.size() == 1) {
            System.out.println("0");
            return;
        }
        System.out.println(comps.size() - 1);
        for (int i = 0; i < comps.size() - 1; i++) {
            int f = comps.get(i);
            int t = comps.get(i + 1);
            Edge e = ex.poll();
            System.out.println((e.a + 1) + " " + (e.b + 1) + " " + (f + 1) + " " + (t + 1));
        }
    }

    static class Edge {

        private int a;
        private int b;

        public Edge(int a, int b) {
            this.a = a;
            this.b = b;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            Edge edge = (Edge) o;

            if (a != edge.a) return false;
            return b == edge.b;

        }

        @Override
        public int hashCode() {
            int result = a;
            result = 31 * result + b;
            return result;
        }
    }

}
