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

public class Main {

    private FastScanner in;
    private PrintWriter out;

    class DisjointSet {
        int size;
        int[] parent;
        int[] rank;
        int count;

        DisjointSet(int size) {
            this.size = size;
            parent = new int[size];
            rank = new int[size];
            for (int i = 0; i < size; i++) {
                parent[i] = i;
                rank[i] = 1;
            }
            count = size;
        }

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

        boolean union(int a, int b) {
            a = find(a);
            b = find(b);
            if (a == b) {
                return false;
            }
            if (rank[a] < rank[b]) {
                int temp = a;
                a = b;
                b = temp;
            }
            parent[b] = a;
            rank[a] += rank[b];
            count--;
            return true;
        }

        int[] getUniqParents() {
            int[] res = new int[count];
            int c = 0;
            Set<Integer> set = new HashSet<>();
            for (int i = 0; i < size; i++) {
                int p = find(i);
                if (!set.contains(p)) {
                    res[c++] = p;
                    set.add(p);
                }
            }
            return res;
        }
    }

    public void solve() throws IOException {
        int size = in.nextInt();
        DisjointSet set = new DisjointSet(size);
        int[] remove1 = new int[size];
        int[] remove2 = new int[size];
        int remove_size = 0;
        for (int i = 0; i < size - 1; i++) {
            int a = in.nextInt();
            int b = in.nextInt();
            if (! set.union(a - 1, b - 1)) {
                remove1[remove_size] = a;
                remove2[remove_size] = b;
                remove_size++;
            }
        }
        int[] parents = set.getUniqParents();
        if (parents.length - 1 != remove_size) {
            throw new IllegalStateException();
        }
        out.println(remove_size);
        for (int i = 0; i < remove_size; i++) {
            out.println(remove1[i] + " " + remove2[i] + " " +
                    (parents[i] + 1) + " " + (parents[i + 1] + 1));
        }
    }

    public void run() {
        try {
            in = new FastScanner(System.in);
            out = new PrintWriter(System.out);
            solve();
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    class FastScanner {

        BufferedReader br;
        StringTokenizer st;

        FastScanner(InputStream is) {
            br = new BufferedReader(new InputStreamReader(is));
        }

        String next() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

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

    public static void main(String[] arg) {
        new Main().run();
    }
}