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.DataInputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;

public class P25D2 {
    public static void main(String[] args) throws IOException {
        InputReader2 ir = new InputReader2();
        PrintWriter pw = new PrintWriter(System.out);

        int n = ir.nextInt();
        ArrayList<ArrayList<Integer>> edges = new ArrayList<>();
        for (int i = 0; i <= n; i++) {
            edges.add(new ArrayList<>());
        }
        for (int i = 1; i < n; i++) {
            int a = ir.nextInt();
            int b = ir.nextInt();
            edges.get(a).add(b);
            edges.get(b).add(a);
        }

        int N = n + 1;
        ArrayList<Integer> components = new ArrayList<>();
        ArrayList<int[]> del = new ArrayList<>();
        del.add(new int[]{-1, -1}); // Place holder so can call del[i] in the print loop to save time
        boolean[] visited = new boolean[N];
        for (int i = 1; i <= n; i++) {
            if (!visited[i]) {
                components.add(i);
                dfs(edges, del, visited, i, -1);
            }
        }
        pw.println(components.size() - 1);
        for (int i = 1; i < components.size(); i++) {
            pw.println(del.get(i)[0] + " " + del.get(i)[1] + " " + components.get(i - 1) + " " + components.get(i));
        }

        pw.close();
    }

    private static void dfs(ArrayList<ArrayList<Integer>> edges, ArrayList<int[]> del, boolean[] visited, int at, int prev) {
        visited[at] = true;

        for (int next : edges.get(at)) {
            if (next != prev && visited[next]) {
                del.add(new int[]{at, next});
            } else if (!visited[next]) {
                dfs(edges, del, visited, next, at);
            }
        }
    }

    private static class InputReader2 {
        private final int BUFFER_SIZE = 1 << 16;
        private final DataInputStream dis;
        private final byte[] buffer;
        private int bufferPointer, bytesRead;

        public InputReader2() {
            dis = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        private int nextInt() throws IOException {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');
            return ret;
        }

        private void fillBuffer() throws IOException {
            bytesRead = dis.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }

        private byte read() throws IOException {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }
    }
}
