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 
// package cf25;

import javafx.util.Pair;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.*;
import java.util.function.IntConsumer;

public class CFD {
    private static final long MOD = 1_000_000_007;
    private static final String INPUT = "7\n" +
            "1 2\n" +
            "2 3\n" +
            "3 1\n" +
            "4 5\n" +
            "5 6\n" +
            "6 7\n";

    private PrintWriter out;
    private FastScanner sc;

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

    public void run() {
        sc = new FastScanner(oj ? System.in : new ByteArrayInputStream(INPUT.getBytes()));
        out = new PrintWriter(System.out);

        long s = System.currentTimeMillis();
        try {
            solve();
        } catch (ExitException ignored) {
        }
        out.flush();
        tr(System.currentTimeMillis() - s + "ms");
    }

    public void solve() {
        int n = sc.nextInt();
        DSU dsu = new DSU(n);

        List<Pair<Integer, Integer>> removed = new ArrayList<>();
        for (int i = 0; i < n - 1; i++) {
            int l = sc.nextInt();
            int r = sc.nextInt();
            int r1 = dsu.find(l);
            int r2 = dsu.find(r);
            if (r1 != r2)
                dsu.parent[r1] = r2;
            else
                removed.add(new Pair<>(l, r));
        }

        Set<Integer> set = new HashSet<>();
        for (int i = 1; i <= n; i++) {
            set.add(dsu.find(i));
        }

        System.out.println(set.size() - 1);
        if (set.size() > 1) {
            Iterator<Integer> it = set.iterator();
            int idx = 0;
            int first = it.next();
            do {
                int lead = it.next();
                Pair<Integer, Integer> pair = removed.get(idx++);
                out.println(pair.getKey() + " " + pair.getValue() + " " + first + " " + lead);
            } while (it.hasNext());
        }
    }

    static class DSU {
        int[] parent;

        DSU(int n) {
            parent = new int[n+1];
            for (int i = 1; i <= n; i++) {
                parent[i] = i;
            }
        }

        int find(int v) {
            if (v == parent[v])
                return v;
            return parent[v] = find(parent[v]);
        }

        void union_sets (int a, int b) {
            a = find(a);
            b = find(b);
            if (a != b)
                parent[b] = a;
        }
    }

    //********************************************************************************************
    //********************************************************************************************
    //********************************************************************************************

    private void times(int n, IntConsumer consumer) {
        for (int i = 0; i < n; i++) {
            try {
                consumer.accept(i);
            } catch (ExitException ignored) {
            }
        }
    }

    private static class ExitException extends RuntimeException {
    }

    private void answer(Object ans) {
        out.println(ans);
        throw new ExitException();
    }

    private static int lowerBound(long[] arr, long key) {
        int lo = 0;
        int hi = arr.length - 1;
        while (lo < hi) {
            int mid = (lo + hi) / 2;
            if (key <= arr[mid]) {
                hi = mid - 1;
            } else {
                lo = mid + 1;
            }
        }
        return arr[lo] < key ? lo + 1 : lo;
    }

    private static int upperBound(long[] arr, long key) {
        int lo = 0;
        int hi = arr.length - 1;
        while (lo < hi) {
            int mid = (lo + hi) / 2;
            if (key >= arr[mid]) {
                lo = mid + 1;
            } else {
                hi = mid;
            }
        }
        return arr[lo] <= key ? lo + 1 : lo;
    }

    private static int ceil(double d) {
        int ret = (int) d;
        return ret == d ? ret : ret + 1;
    }

    private static int round(double d) {
        return (int) (d + 0.5);
    }

    private static int gcd(int a, int b) {
        BigInteger b1 = BigInteger.valueOf(a);
        BigInteger b2 = BigInteger.valueOf(b);
        BigInteger gcd = b1.gcd(b2);
        return gcd.intValue();
    }

    private static long gcd(long a, long b) {
        BigInteger b1 = BigInteger.valueOf(a);
        BigInteger b2 = BigInteger.valueOf(b);
        BigInteger gcd = b1.gcd(b2);
        return gcd.longValue();
    }

    private int[] readIntArray(int n) {
        int[] res = new int[n];
        for (int i = 0; i < n; i++) {
            res[i] = sc.nextInt();
        }
        return res;
    }

    private long[] readLongArray(int n) {
        long[] res = new long[n];
        for (int i = 0; i < n; i++) {
            res[i] = sc.nextLong();
        }
        return res;
    }

    @SuppressWarnings("unused")
    static class FastScanner {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;

        FastScanner(InputStream stream) {
            this.stream = stream;
        }

        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++];
        }

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

        boolean isEndline(int c) {
            return c == '\n' || c == '\r' || c == -1;
        }

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

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public String next() {
            int c = read();
            while (isSpaceChar(c)) c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public String nextLine() {
            int c = read();
            while (isEndline(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isEndline(c));
            return res.toString();
        }

    }

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

    private void tr(Object... o) {
        if (!oj) System.out.println(Arrays.deepToString(o));
    }
}