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.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.BufferedReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Omar-Handouk
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastReader in = new FastReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        TaskD solver = new TaskD();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskD {
        public void solve(int testNumber, FastReader in, OutputWriter out) {
            int n = in.nextInt();
            Disjoint_Set set = new Disjoint_Set(n);

            //Target is to make One set only.
            ArrayList<Road> redundent = new ArrayList<>();

            for (int i = 1; i <= n - 1; ++i) {
                int a = in.nextInt() - 1;
                int b = in.nextInt() - 1;

                if (!set.sameSet(a, b))
                    set.union(a, b);
                else {
                    redundent.add(new Road(a, b));
                }
            }

            if (set.getSetCount() == 1)
                out.println(0);
            else {
                int days = 0;
                StringBuilder build = new StringBuilder();
                while (set.getSetCount() != 1) {
                    Road road = redundent.remove(0);
                    for (int i = 0; i < n; ++i) {
                        //System.err.println(road.from + " " + i);
                        //System.err.println(set.sameSet(road.from, i));
                        if (!set.sameSet(road.from, i)) {
                            build.append((road.from + 1) + " " + (road.to + 1) + " " + (road.from + 1) + " " + (i + 1) + "\n");
                            set.union(road.from, i);
                            ++days;
                            break;
                        }
                    }
                }
                out.println(days);
                out.println(build);
            }
        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void println(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

        public void println(int i) {
            writer.println(i);
        }

    }

    static class Disjoint_Set {
        private int Parent[];
        private int setSize[];
        private int Rank[];
        private int setCount;

        public Disjoint_Set(int Members) {
            Parent = new int[Members];
            setSize = new int[Members];
            Rank = new int[Members];
            setCount = Members;
            initialize(Members);
        }

        private void initialize(int Members) {
            for (int i = 0; i < Members; ++i) {
                Parent[i] = i;
                setSize[i] = 1;
            }
        }

        public int findSet(int Member) {
            if (Member == Parent[Member])
                return Member;

            return Parent[Member] = findSet(Parent[Member]);
        }

        public boolean sameSet(int Member_1, int Member_2) {
            return findSet(Member_1) == findSet(Member_2);
        }

        public void union(int Member_1, int Member_2) {
            if (sameSet(Member_1, Member_2))
                return;

            setCount = setCount - 1;

            int Parent_1 = findSet(Member_1);
            int Parent_2 = findSet(Member_2);

            if (Rank[Parent_1] > Rank[Parent_2]) {
                Parent[Parent_2] = Parent_1;
                setSize[Parent_1] = setSize[Parent_1] + setSize[Parent_2];
            } else if (Rank[Parent_1] < Rank[Parent_2]) {
                Parent[Parent_1] = Parent_2;
                setSize[Parent_2] = setSize[Parent_2] + setSize[Parent_1];
            } else {
                Parent[Parent_2] = Parent_1;
                setSize[Parent_1] = setSize[Parent_1] + setSize[Parent_2];
                Rank[Parent_1] = Rank[Parent_1] + 1;
            }
        }

        public int getSetCount() {
            return setCount;
        }

    }

    static class Road {
        int from;
        int to;

        Road(int a, int b) {
            from = a;
            to = b;
        }

    }

    static class FastReader {
        BufferedReader bufferedReader;
        StringTokenizer stringTokenizer;

        public FastReader(InputStream stream) {
            bufferedReader = new BufferedReader(new InputStreamReader(stream));
        }

        public String next() {
            while (stringTokenizer == null || !stringTokenizer.hasMoreTokens()) {
                try {
                    stringTokenizer = new StringTokenizer(bufferedReader.readLine());
                } catch (IOException error) {
                    error.printStackTrace();
                }
            }

            return stringTokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(this.next());
        }

    }
}

