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.*;
public class NotBerland {

	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		int size=sc.nextInt();
		ArrayList<Edge> lis=new ArrayList<Edge>();
		for(int w=0;w<size-1;w++)
			lis.add(new Edge(sc.nextInt()-1, sc.nextInt()-1));
		int[] arr=new int[size];
		for(int x=0;x<size;x++)
			arr[x]=x;
		ArrayList<Edge> dis=new ArrayList<Edge>();
		int tree=size-1;
		for(int x=0;x<lis.size();x++)
		{
			Edge tem=lis.get(x);
			if(union(arr,tem))
			{
				tree--;
			}
			else dis.add(tem);
		}
		System.out.println(dis.size());
		while(!dis.isEmpty())
		{
			for(int x=0;x<size && !dis.isEmpty();x++)
				for(int y=0;y<size && !dis.isEmpty();y++)
				{
					if(union(arr,new Edge(x, y)))
					{
						Edge e=dis.remove(0);
						Edge tem=new Edge(x, y);
						print(e,tem);
					}
				}
		}
		
	}
	public static void print(Edge a,Edge b)
	{
		int aa=a.a+1;
		int ab=a.b+1;
		int ba=b.a+1;
		int bb=b.b+1;
		System.out.println(aa+" "+ab+" "+ba+" "+bb);
	}
	public static boolean union(int[] arr,Edge e)
	{
		int pa=find(arr,e.a);
		int pb=find(arr,e.b);
		if(pa==pb)return false;
		arr[e.a]=pb;
		return true;
	}
	public static int find(int[] arr,int x)
	{
		if(arr[x]!=arr[arr[x]])arr[x]=find(arr,arr[x]);
		return arr[x];
	}
}
class Edge
{
	int a,b;
	public Edge(int x,int y)
	{
		a=x;
		b=y;
	}
}
