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.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.List;

import static java.lang.Math.abs;
import static java.lang.Math.max;
import static java.lang.Math.min;

public class BerlandDsu {
    static class Task {
        int n;
        List<Integer>[] adj;
        int[] state;
        List<int[]> ans;
        int[] link;
        int[] sz;

        void init() {
            link = new int[n + 1];
            Arrays.setAll(link, i -> i);
            sz = new int[n + 1];
            Arrays.fill(sz, 1);
        }

        int find(int u) {
            while (u != link[u]) {
                u = link[u];
            }
            return u;
        }

        boolean same(int u, int v) {
            return find(u) == find(v);
        }

        void unite(int u, int v) {
            u = find(u);
            v = find(v);
            assert (u != v);
            if (sz[u] > sz[v]) {
                int temp = u;
                u = v;
                v = temp;
            }
            link[u] = v;
            sz[v] += sz[u];
        }

        void dfs(int u, int par) {
            state[u] = 1;
            for (int v : adj[u]) {
                if (v == par) {
                    continue;
                }
                if (state[v] == 1) {
                    // found cycle
                    for (int i = 1; i <= n; i++) {
                        if (!same(u, i)) {
                            unite(u, i);
                            ans.add(new int[]{u, v, u, i});
                            break;
                        }
                    }
                } else if (state[v] == 0) {
                    dfs(v, u);
                }
            }
            state[u] = 2;
        }

        public void solve() {
            n = ni();
            adj = new List[n + 1];
            Arrays.setAll(adj, i -> new ArrayList<>());
            init();
            for (int it = 0; it < n - 1; it++) {
                int u = ni(), v = ni();
                adj[u].add(v);
                adj[v].add(u);
                if (!same(u, v)) {
                    unite(u, v);
                }
            }

            ans = new ArrayList<>();
            state = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                if (state[i] == 0) {
                    dfs(i, -1);
                }
            }

            out.println(ans.size());
            for (int[] a : ans) {
                for (int i = 0; i < 4; i++) {
                    out.print(a[i] + " ");
                }
                out.println();
            }
        }

    }


    /*==================================================================================================================*/

    // Startup
    static String INPUT = "";
    static PrintWriter out;

    public static void main(String[] args) {
        // is = new ByteArrayInputStream(INPUT.getBytes());
        is = System.in;
        out = new PrintWriter(System.out);
        long s = System.currentTimeMillis();
        Task solver = new Task();
        int tt = 1;
        // tt = ni();
        for (int i = 1; i <= tt; i++) {
            solver.solve();
        }
        out.flush();
        tr(System.currentTimeMillis() - s + "ms");
    }


    // Input/output
    static InputStream is;
    private static final byte[] inbuf = new byte[1024];
    public static int lenbuf = 0, ptrbuf = 0;

    private static int readByte() {
        if (lenbuf == -1) throw new InputMismatchException();
        if (ptrbuf >= lenbuf) {
            ptrbuf = 0;
            try {
                lenbuf = is.read(inbuf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (lenbuf <= 0) return -1;
        }
        return inbuf[ptrbuf++];
    }

    private static boolean isSpaceChar(int c) {
        return !(c >= 33 && c <= 126);
    }

    private static int skip() {
        int b;
        while ((b = readByte()) != -1 && isSpaceChar(b)) ;
        return b;
    }

    private static double nd() {
        return Double.parseDouble(ns());
    }

    private static char nc() {
        return (char) skip();
    }

    private static String ns() {
        int b = skip();
        StringBuilder sb = new StringBuilder();
        while (!(isSpaceChar(b))) { // when nextLine, (isSpaceChar(b) && b != ' ')
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    private static char[] ns(int n) {
        char[] buf = new char[n];
        int b = skip(), p = 0;
        while (p < n && !(isSpaceChar(b))) {
            buf[p++] = (char) b;
            b = readByte();
        }
        return n == p ? buf : Arrays.copyOf(buf, p);
    }

    private static char[][] nm(int n, int m) {
        char[][] map = new char[n][];
        for (int i = 0; i < n; i++) map[i] = ns(m);
        return map;
    }

    private static int[] na(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++) a[i] = ni();
        return a;
    }

    private static int ni() {
        int num = 0, b;
        boolean minus = false;
        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')) ;
        if (b == '-') {
            minus = true;
            b = readByte();
        }

        while (true) {
            if (b >= '0' && b <= '9') {
                num = num * 10 + (b - '0');
            } else {
                return minus ? -num : num;
            }
            b = readByte();
        }
    }

    private static long nl() {
        long num = 0;
        int b;
        boolean minus = false;
        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')) ;
        if (b == '-') {
            minus = true;
            b = readByte();
        }

        while (true) {
            if (b >= '0' && b <= '9') {
                num = num * 10 + (b - '0');
            } else {
                return minus ? -num : num;
            }
            b = readByte();
        }
    }

    // Debug
    private static final boolean oj = System.getProperty("ONLINE_JUDGE") != null;

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