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 Solution1{

	static class Road{
		int u,v;
		Road(int u,int v){
			this.u=u;
			this.v=v;
		}
		public String toString(){
			return u+" "+v;
		}
	}

	public static void dfs(int root, int parent, LinkedList<Integer> adj[], boolean visited[], HashSet<String> extra){

		visited[root]=true;
		for(int item:adj[root])
			if(item!=parent){
				if(visited[item]){
					if(!extra.contains(root+" "+item) && !extra.contains(item+" "+root))
						extra.add(root+" "+item);
				}
				else
					dfs(item, root, adj, visited, extra);
			}
	}

	public static void main(String args[])throws IOException{

		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

		int n=Integer.parseInt(br.readLine());

		LinkedList<Integer> adj[]=new LinkedList[n+1];

		for(int i=0;i<=n;i++)
			adj[i]=new LinkedList<>();

		for(int i=0;i<n-1;i++){
			String input[]=br.readLine().split("\\s");
			int u=Integer.parseInt(input[0]),
				v=Integer.parseInt(input[1]);

			adj[u].add(v);
			adj[v].add(u);
		}

		
		boolean visited[]=new boolean[n+1];
		HashSet<String> extra=new HashSet<>();
		ArrayList<Integer> forest=new ArrayList<>();

		for(int i=1;i<=n;i++)
			if(!visited[i]){
				dfs(i, 0, adj, visited, extra);
				forest.add(i);
			}

		System.out.println(extra.size());
		//System.out.println(extra);
		int i=1;
		for(String item:extra){
			System.out.println(item+" "+forest.get(i-1)+" "+forest.get(i));
			i+=1;
		}
		
	}
}