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.io.*;
import java.util.*;

import static java.lang.Math.*;

public class ConnectCountries {
	static StringTokenizer _stk; static BufferedReader input; static PrintWriter output; 
	static String next(){return _stk.nextToken();} static int nextInt(){return Integer.parseInt(next());}
	static String readln()throws IOException {String l=input.readLine();_stk=l==null?null:new StringTokenizer(l," ");return l;}
	public static void main(String[] args) throws IOException {
		input = new BufferedReader(new InputStreamReader(System.in)); 
		output = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
		new ConnectCountries();
		output.close();
	}

	ConnectCountries() throws IOException {
		readln();
		N = nextInt();
		grafo = new LinkedList[N+1];
		//HashSet<Edge> initEdges = new HashSet<Edge>();
		for (int i = 0; i < N-1; i++) {
			readln();
			int a = nextInt()-1, b = nextInt()-1;
			if(grafo[a]==null) grafo[a]=new LinkedList<Integer>();
			if(grafo[b]==null) grafo[b]=new LinkedList<Integer>();
			grafo[a].add(b);
			grafo[b].add(a);
			//initEdges.add(new Edge(a,b));
		}
		
		//MST
		padre = new int[N];
		Arrays.fill(padre,-1);
		for (int i = 0; i < N; i++) {
			if(padre[i]==-1) {
				dfs(i,i);
			}
		}
		
		ArrayList<Integer> heads = new ArrayList<Integer>();
		for (int i = 0; i < N; i++) {
			if(padre[i]==i) heads.add(i);
		}
		
		output.println(heads.size()-1);
		Iterator<Edge> it = deleted.iterator();
		for(int i=1; i<heads.size(); i++) {
			Edge del = it.next();
			
			output.println((del.x+1)+" "+(del.y+1)+" "+(heads.get(0)+1)+" "+(heads.get(i)+1));
		}
	}
	
	int padre[];
	Set<Edge> deleted = new HashSet<Edge>();
	void dfs(int node, int pdr) {
		padre[node]=pdr;
		if(grafo[node]==null) return;
		for(int neigh : grafo[node]) {
			if(padre[neigh]==-1)
				dfs(neigh,node);
			else 
				deleted.add(new Edge(node,neigh));
		}
	}
	
	class Edge { 
		public Edge(int x, int y) {
			this.x = min(x,y);
			this.y = max(x,y);
		}

		int x, y; 
		
		public int hashCode() {
			return x*1000+y;
		}
		public boolean equals(Object o) {
			Edge e = (Edge)o;
			return x==e.x && y==e.y;
		}
	}
	
	int N;
	LinkedList<Integer>[] grafo;
}