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.Vector;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Stack;
import java.io.InputStream;

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

    static class TaskD {
        Stack<Integer> rU;
        Stack<Integer> rV;
        int[] parent;

        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            init(n);
            for (int i = 1; i < n; i++) {
                int u = in.nextInt();
                int v = in.nextInt();
                union(u, v);
            }
            Stack<Integer> leader = new Stack<>();
            boolean[] used = new boolean[n + 1];
            for (int i = 1; i <= n; i++) {
                if (!used[find(i)]) {
                    used[find(i)] = true;
                    leader.push(find(i));
                }
            }
            out.println(rU.size());
            if (leader.size() > 0) {
                int lastLeader = leader.pop();
                while (!leader.isEmpty()) {
                    int cur = leader.pop();
                    out.println(rU.pop() + " " + rV.pop() + " " + lastLeader + " " + cur);
                    lastLeader = cur;
                }
            }
        }

        void init(int n) {
            parent = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                parent[i] = i;
            }
            rU = new Stack<>();
            rV = new Stack<>();
        }

        int find(int node) {
            while (parent[node] == node) {
                return node;
            }
            return parent[node] = find(parent[node]);
        }

        void union(int n1, int n2) {
            int n1r = find(n1);
            int n2r = find(n2);
            if (n1r == n2r) {
                rU.push(n1);
                rV.push(n2);
            } else {
                parent[n1r] = n2r;
            }
        }

    }

    static class InputReader {
        private BufferedReader reader;
        private StringTokenizer stt;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream));
        }

        public String nextLine() {
            try {
                return reader.readLine();
            } catch (IOException e) {
                return null;
            }
        }

        public String next() {
            while (stt == null || !stt.hasMoreTokens()) {
                stt = new StringTokenizer(nextLine());
            }
            return stt.nextToken();
        }

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

    }
}

