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 P25D
{
	public static void main(String[]args) throws Exception
	{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(br.readLine());
		ArrayList<Node> graph = new ArrayList<Node>(); 
		for(int i=0; i<n; i++)
			graph.add(new Node(i+1)); 
		
		ArrayDeque<Edge> krus = new ArrayDeque<Edge>(); 
		
		for(int i=0; i<n-1; i++)
		{
			String input = br.readLine();
			StringTokenizer tk = new StringTokenizer(input);
			Node s = graph.get(Integer.parseInt(tk.nextToken())-1);
			Node d = graph.get(Integer.parseInt(tk.nextToken())-1); 
			
			krus.offer(new Edge(s,d));
		}
		ArrayList<Edge> toDelete = new ArrayList<Edge>(); 
		
		while(!krus.isEmpty())
		{
			Edge temp = krus.poll();
			Node s = temp.s;
			Node d = temp.d; 
			if(find(s)!=find(d))
				union(s,d); 
			else
				toDelete.add(temp); 
		}
		
		System.out.println(toDelete.size());
		ArrayList<Node> roots = new ArrayList<Node>();
		for(int i=0;i<graph.size();i++)
		{
			Node temp = graph.get(i);
			if(temp.par==temp)
				roots.add(temp); 
		}
		int num = 0; 
		StringBuilder sb = new StringBuilder(); 
		for(int i=0; i<toDelete.size();i++)
		{
			Edge hold = toDelete.get(i); 
			sb.append(hold.s.name + " " + hold.d.name + " ");
			sb.append(roots.get(num).name + " " + roots.get(num+1).name +"\n");
			num++;
		}
		System.out.print(sb);
	}
	
	public static Node find(Node a)
	{
		if(a!=a.par) a.par=find(a.par);
		return a.par; 
	}
	
	public static void union(Node a, Node b)
	{
		Node x = find(a);
		Node y = find(b);
		x.par = y.par; 
	}
}

class Node
{
	int name; 
	Node par; 
	public Node(int name)
	{
		par = this;
		this.name=name; 
	}
}

class Edge
{
	Node s,d;
	public Edge(Node s, Node d)
	{
		this.s=s;
		this.d=d; 
	}
}