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.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Objects;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

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

    static class DRoadsNotOnlyInBerland {
        public void solve(int testNumber, InputReader c, OutputWriter w) {
            int n = c.readInt();
            UnionFind dsu = new UnionFind(n);
            ArrayList<Pair<Integer, Integer>> aa = new ArrayList<>();
            for (int i = 0; i < n - 1; i++) {
                int u = c.readInt() - 1, v = c.readInt() - 1;
                if (dsu.connected(u, v)) {
                    aa.add(new Pair<>(u, v));
                } else {
                    dsu.unify(u, v);
                }
            }

            w.printLine(aa.size());
            ArrayList<Integer> comp = new ArrayList<>();
            HashSet<Integer> hh = new HashSet<>();
            for (int i = 0; i < n; i++) {
                if (!hh.contains(dsu.find(i))) {
                    hh.add(dsu.find(i));
                    comp.add(dsu.find(i));
                }
            }

            for (int i = 0; i < comp.size() - 1; i++) {
                w.printLine(aa.get(i).first + 1, aa.get(i).second + 1, comp.get(i) + 1, comp.get(comp.size() - 1) + 1);
            }


        }

    }

    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 printLine(Object... objects) {
            print(objects);
            writer.println();
        }

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

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

    }

    static class UnionFind {
        private int size;
        private int[] sz;
        private int[] id;
        private int numComponents;

        public UnionFind(int size) {

            if (size <= 0) {
                throw new IllegalArgumentException("Size <= 0 is not allowed");
            }

            this.size = numComponents = size;
            sz = new int[size];
            id = new int[size];

            for (int i = 0; i < size; i++) {
                id[i] = i;
                sz[i] = 1;
            }
        }

        public int find(int p) {

            int root = p;
            while (root != id[root]) {
                root = id[root];
            }

            while (p != root) {
                int next = id[p];
                id[p] = root;
                p = next;
            }

            return root;
        }

        public boolean connected(int p, int q) {
            return find(p) == find(q);
        }

        public void unify(int p, int q) {

            int root1 = find(p);
            int root2 = find(q);

            if (root1 == root2) {
                return;
            }

            if (sz[root1] < sz[root2]) {
                sz[root2] += sz[root1];
                id[root1] = root2;
            } else {
                sz[root1] += sz[root2];
                id[root2] = root1;
            }
            numComponents--;
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int readInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }

    static class Pair<U, V> implements Comparable<Pair<U, V>> {
        public final U first;
        public final V second;

        public Pair(U first, V second) {
            this.first = first;
            this.second = second;
        }

        public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }
            Pair<?, ?> pair = (Pair<?, ?>) o;
            return first.equals(pair.first) &&
                    second.equals(pair.second);
        }

        public int hashCode() {
            return Objects.hash(first, second);
        }

        public String toString() {
            return "(" + first + "," + second + ")";
        }

        public int compareTo(Pair<U, V> o) {
            int value = ((Comparable<U>) first).compareTo(o.first);
            if (value != 0) {
                return value;
            }
            return ((Comparable<V>) second).compareTo(o.second);
        }

    }
}

