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.ArrayList;
import java.util.Scanner;

public class Main {
	
	static int time=0;
	
	public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		
		int n=s.nextInt();
		
		ArrayList<Integer>[] arr=new ArrayList[n+1];
		
		for(int i=0;i<n+1;i++)
		{
			ArrayList<Integer> list=new ArrayList<>();
			arr[i]=list;
		}
		
		for(int i=0;i<n-1;i++)
		{
			int a=s.nextInt();
			int b=s.nextInt();
			
			arr[a].add(b);
			arr[b].add(a);
		}
		
		int[] visited=new int[n+1];
		int[] lotime=new int[n+1];
		int[] distime=new int[n+1];
		
		ArrayList<Integer> head=new ArrayList<>();
		ArrayList<pair> list=new ArrayList<>();
		
		for(int i=1;i<=n;i++)
		{
			if(visited[i]==0)
			{
				head.add(i);
				dfs(arr,visited,i,list,distime,lotime,-1);
			}
		}
		
		if(head.size()==1)
		{
			System.out.println(0);
		}
		else
		{
			System.out.println(head.size()-1);
			
			for(int i=1;i<head.size();i++)
			{
				pair p=list.remove(list.size()-1);
				System.out.println(p.a+" "+p.b+" "+head.get(i-1)+" "+head.get(i));
			}
		}
		
	}
	
	public static void dfs(ArrayList<Integer>[] arr,int[] visited,int curr,ArrayList<pair> list,int[] distime,int[] lotime,int parent)
	{
		visited[curr]=1;
		
		time++;
		
		distime[curr]=time;
		lotime[curr]=time;
		
		for(int i=0;i<arr[curr].size();i++)
		{
			int now=arr[curr].get(i);
			
			if(visited[now]==0)
			{
				dfs(arr,visited,now,list,distime,lotime,curr);
				
				if(lotime[now]<=distime[curr])
				{
					lotime[curr]=Math.min(lotime[curr], lotime[now]); 
					
					pair p=new pair(curr,now);
					list.add(p);
				}
			}
			else if(now!=parent)
			{
				lotime[curr]=Math.min(lotime[curr], lotime[now]); 
			}
		}
		
	}
	
}

class pair
{
	int a;
	int b;
	
	public pair(int a,int b)
	{
		this.a=a;
		this.b=b;
	}
}