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 implements Runnable {
		int[] p, r;
                private void init(int N){
                        p = new int[N];
                        r = new int[N];
                        for(int i = 0; i < N; i++) p[i] = i;
                }
                
                private int get(int i){
                        if(p[i] != i)
                                p[i] = get(p[i]);
                        return p[i];
                }
                
                private void union(int a, int b){
                        a = get(a);
                        b = get(b);
                        if(a == b)
                                return;
                        if(r[a] == r[b]) r[a]++;
                        if(r[a] >= r[b]){
                                p[b] = a;
                        }
                        else p[a] = b;
                }
                
	public void solve() throws IOException {
		int N = nextInt();
                init(N);
                
                List<Integer> destroy = new ArrayList<>();
                List<Integer> build = new ArrayList<>();
                
                for(int i = 0; i < N-1 ; i++){
                        int a = nextInt() - 1;
                        int b = nextInt() - 1;
                        if(get(a) == get(b)){
                                destroy.add(a + 1);
                                destroy.add(b + 1);
                        }
                        else{
                                union(a, b);
                        }
                }
                
                for(int i = 0; i < N; i++){
                        for(int j = i + 1; j < N; j++){
                                if(get(i) != get(j)){
                                        build.add(i + 1);
                                        build.add(j + 1);
                                        union(i, j);
                                }
                        }
                }
                
                int s = build.size();
                System.out.println(s / 2);
                StringBuilder sb = new StringBuilder();
                for(int i = 0; i < s; i += 2){
                        sb.append(destroy.get(i) + " " + destroy.get(i + 1) + " ");
                        sb.append(build.get(i) + " " + build.get(i + 1) + "\n");
                }
	System.out.print(sb.toString());
        }
        

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

	public void run() {
		try {
			in = new BufferedReader(new InputStreamReader(System.in));
			tok = null;
			solve();
			in.close();
		} catch (IOException e) {
			System.exit(0);
		}
	}

	public String nextToken() throws IOException {
		while (tok == null || !tok.hasMoreTokens()) {
			tok = new StringTokenizer(in.readLine());
		}
		return tok.nextToken();
	}

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

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

	public double nextDouble() throws IOException {
		return Double.parseDouble(nextToken());
	}

	BufferedReader in;
	StringTokenizer tok;
}