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.*;
import static java.lang.System.*;
import static java.lang.Integer.*;

public class CF25D{
	public static void main(String[] args)throws Exception{
		BufferedReader sc = new BufferedReader(new InputStreamReader(in));
		
		int n = parseInt(sc.readLine());
		ArrayList<Node> nodes = new ArrayList<Node>();
		for(int i=0; i<n; i++){
			nodes.add(new Node(i+1));
		}
		
		ArrayList<Edge> edges = new ArrayList<Edge>();
		for(int i=1; i<n; i++){
			StringTokenizer st = new StringTokenizer(sc.readLine());
			Node a = nodes.get(parseInt(st.nextToken())-1);
			Node b = nodes.get(parseInt(st.nextToken())-1);
			Edge e = new Edge(a, b);
			edges.add(e);
			a.cons.add(e);
			b.cons.add(e);
		}
		
		ArrayList<HashSet<Node>> sets = new ArrayList<HashSet<Node>>();
		for(int i=0; i<n; i++){
			if(nodes.get(i).vis) continue;
			HashSet<Node> set = new HashSet<Node>();
			sets.add(set);
			ArrayDeque<Node> queue = new ArrayDeque<Node>();
			queue.offer(nodes.get(i));
			nodes.get(i).vis = true;
			while(queue.size()>0){
				Node no = queue.poll();
				set.add(no);
				for(int j=0; j<no.cons.size(); j++){
					if(no.cons.get(j).other(no) != no.par){
						if(!no.cons.get(j).other(no).vis){
							queue.offer(no.cons.get(j).other(no));
							no.cons.get(j).other(no).vis = true;
							no.cons.get(j).other(no).par = no;
						}else{
							no.cons.get(j).pick = false;
						}
					}
				}
			}
		}
		
		int cur = 0;
		StringBuilder sb = new StringBuilder();
		for(int i=0; i<edges.size(); i++){
			if(edges.get(i).pick) continue;
			
			Node a = sets.get(cur).iterator().next();
			Node b = sets.get(cur+1).iterator().next();
			cur++;
			sb.append(edges.get(i).a.id + " " + edges.get(i).b.id + " " + a.id + " " + b.id + "\n");
		}
		
		out.println(cur);
		out.print(sb);
	}
}

class Node{
	ArrayList<Edge> cons = new ArrayList<Edge>();
	int id;
	Node par;
	boolean vis;
	public Node(int n){
		id = n;
		vis = false;
	}
}

class Edge{
	Node a, b;
	boolean pick;
	public Edge(Node s, Node e){
		a = s;
		b = e;
		pick = true;
	}
	public Node other(Node n){
		if(n == a) return b;
		return a;
	}
}