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.util.*;
import java.io.*;

public class Main {

    StringTokenizer st;
    BufferedReader br;
    PrintWriter out;

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

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

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

    int parent[] = new int[1001];

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

    boolean union(int a, int b) {
        if (find(a) == find(b)) return false;
        parent[find(a)] = parent[find(b)];
        return true;
    }

    void solve() throws IOException {
        int n = nextInt();
        for (int i = 0; i < n; i++)
            parent[i] = i;
        int l[] = new int[n + 1];
        int r[] = new int[n + 1];
        int head = 0;
        for (int i = 0; i < n - 1; i++) {
            int a = nextInt(), b = nextInt();
            a--;
            b--;
            if (!union(a, b)) {
                l[head] = a;
                r[head] = b;
                head++;
            }
        }
        out.println(head);
        int tail = 0;
        for (int i = 0; i < n; i++) {
            if (union(0, i)) {
                out.println((l[tail] + 1) + " " + (r[tail] + 1) + " " + 1 + " " + (i + 1));
                tail++;
            }
        }
    }

    private void run() throws IOException {
        br = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);
        solve();
        br.close();
        out.close();
    }

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