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.*;
import java.util.*;

public class Q25D implements Runnable {
    private void solve() throws IOException {
        int n = nextInt();
        List<Integer>[] gg = new List[n];
        for (int i = 0; i < gg.length; i++) gg[i] = new ArrayList<Integer>();
        for (int i = 0; i < n - 1; i++) {
            int from = nextInt() - 1;
            int to = nextInt() - 1;
            gg[from].add(to);
            gg[to].add(from);
        }
        List<Integer> trees = new ArrayList<Integer>();
        Set<Integer> notUsed = new HashSet<Integer>();
        Set<Road> toDelete = new HashSet<Road>();
        for (int i = 0; i < n; i++) {
            notUsed.add(i);
        }

        while (!notUsed.isEmpty()) {
            Iterator<Integer> iterator = notUsed.iterator();
            int root = iterator.next();
            iterator.remove();
            boolean[] used = new boolean[n];
            int[] parent = new int[n];
            Queue<Integer> bfs = new ArrayDeque<Integer>();
            bfs.add(root);
            trees.add(root);
            while (!bfs.isEmpty()) {
                int from = bfs.poll();
                used[from] = true;
                notUsed.remove(from);
                for (int i = 0; i < gg[from].size(); i++) {
                    int to = gg[from].get(i);
                    parent[to] = from;
                    if (used[to]) {
                        if (to != parent[from]) {
                            toDelete.add(new Road(from,to));
                        }
                    } else {
                        bfs.add(to);
                    }
                }
            }
        }

        Iterator<Integer> iterator = trees.iterator();
        int z = trees.size();
        int from = 0;
        if (iterator.hasNext()) {
            from = iterator.next();
            iterator.remove();
        }
        writer.println(z-1);
        Road[] roads = toDelete.toArray(new Road[0]);
        for (int i = 0; i < z-1; i++) {
            Road road= roads[i];
            int to = iterator.next();
            iterator.remove();
            writer.println((road.from+1) + " " + (road.to+1) + " " + (from+1) + " " + (to+1));
            from = to;
        }
    }


    private class Road {
        int from;
        int to;

        private Road(int from, int to) {
            if (from < to) {
                this.from = from;
                this.to = to;
            } else {
                this.to = from;

                this.from = to;

            }

        }

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

            Road road = (Road) o;

            if (from != road.from) return false;
            if (to != road.to) return false;

            return true;
        }

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

    public static void main(String[] args) {
        new Q25D().run();
    }

    BufferedReader reader;
    StringTokenizer tokenizer;
    PrintWriter writer;

    public void run() {
        try {

            boolean oj = System.getProperty("ONLINE_JUDGE") != null;

            reader = oj ? new BufferedReader(new InputStreamReader(System.in)) :
                    new BufferedReader(new FileReader(new File("input.txt")));


            tokenizer = null;
            writer = new PrintWriter(System.out);
            long start = System.currentTimeMillis();
            solve();
            reader.close();
            if (!oj) {
                writer.print("\nTime:" + (System.currentTimeMillis() - start));
            }
            writer.close();
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    int nextInt() throws IOException {
        return Integer.parseInt(nextToken());
    }

    long nextLong() throws IOException {
        return Long.parseLong(nextToken());
    }

    double nextDouble() throws IOException {
        return Double.parseDouble(nextToken());
    }

    String nextToken() throws IOException {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            tokenizer = new StringTokenizer(reader.readLine());
        }
        return tokenizer.nextToken();
    }
}
