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.PrintWriter;
import java.util.Iterator;
import java.util.Set;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.InputStream;

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

    static class TaskD {
        public void solve(int testNumber, QuickScanner in, PrintWriter out) {
            int n = in.nextInt();
            DisjointSet ds = new DisjointSet(n);
            List<Integer>[] tree = CollectionsUtils.arrayOfListInteger(n);
            List<IntPair> ll = new ArrayList<>();
            for (int i = 1; i < n; i++) {
                int u = in.nextInt() - 1;
                int v = in.nextInt() - 1;
                if (!ds.connected(u, v)) {
                    tree[u].add(v);
                    tree[v].add(u);
                    ds.connect(u, v);
                } else {
                    ll.add(new IntPair(u, v));
                }
            }
            if (ll.isEmpty()) out.println(0);
            else {
                Set<Integer> roots = new HashSet<>();
                for (int i = 0; i < n; i++) {
                    roots.add(ds.root(i));
                }
                int last = -1;
                out.println(roots.size() - 1);
                Iterator<IntPair> it = ll.iterator();
                for (int r : roots) {
                    if (last != -1) {
                        IntPair pair = it.next();
                        out.printf("%d %d %d %d\n", pair.first + 1, pair.second + 1, r + 1, last + 1);
                    }
                    last = r;
                }
            }
        }

    }

    static class IntPair implements Comparable<IntPair> {
        public final int first;
        public final int second;

        public IntPair() {
            this(0, 0);
        }

        public IntPair(int u, int v) {
            this.first = u;
            this.second = v;
        }


        public int hashCode() {
            int hu = (int) (first ^ (first >>> 32));
            int hv = (int) (second ^ (second >>> 32));
            return 31 * hu + hv;
        }


        public boolean equals(Object obj) {
            IntPair pair = (IntPair) obj;
            return pair.first == first && pair.second == second;
        }


        public int compareTo(IntPair o) {
            return first == o.first ? Integer.compare(second, o.second) : Integer.compare(first, o.first);
        }


        public String toString() {
            return String.format("(%d,%d)", first, second);
        }

    }

    static class QuickScanner {
        BufferedReader br;
        StringTokenizer st;
        InputStream is;

        public QuickScanner(InputStream stream) {
            is = stream;
            br = new BufferedReader(new InputStreamReader(stream), 32768);
        }

        public String nextToken() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return st.nextToken();
        }

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

    }

    static class CollectionsUtils {
        public static List<Integer>[] arrayOfListInteger(int n) {
            List<Integer>[] ret = new List[n];
            for (int i = 0; i < n; i++) {
                ret[i] = new ArrayList<>();
            }
            return ret;
        }

    }

    static class DisjointSet {
        int[] ds;
        int[] rank;
        int count;

        public DisjointSet(int n) {
            ds = new int[n];
            rank = new int[n];
            count = n;
            for (int i = 0; i < n; i++) {
                ds[i] = i;
            }
        }

        public int root(int p) {
            if (p == ds[p]) return p;
            return (ds[p] = root(ds[p]));
        }

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

        public void connect(int p, int q) {
            int rp = root(p);
            int rq = root(q);
            if (rp == rq) return;
            if (rank[rp] < rank[rq]) {
                ds[rp] = rq;
            } else if (rank[rp] > rank[rq]) {
                ds[rq] = rp;
            } else {
                ds[rq] = rp;
                rank[rp]++;
            }
            count--;
        }

    }
}

