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 Main{

	static int mod=(int)1e8;
	
	public static void main(String[] args) throws IOException {
		PrintWriter out=new PrintWriter(System.out);
		Reader in=new Reader(System.in);	
		int ts=1;
//		ts=in.nextInt();
		outer: while(ts-->0) {
			
			int n=in.nextInt();
			ArrayList<edge> toClose=new ArrayList<>(), toBuild=new ArrayList<>();
			dsu uf=new dsu(n);
			for(int i=0; i<n; ++i) uf.make(i);
			for(int i=0; i<n-1; ++i) {
				int x=in.nextInt()-1;
				int y=in.nextInt()-1;
				if(uf.find(x)==uf.find(y)) toClose.add(new edge(x+1,y+1));
				else uf.union(x, y);
			}
			
			for(int i=1; i<n; ++i) {
				if(uf.find(0)!=uf.find(i)) { 
					toBuild.add(new edge(1,i+1));
					uf.union(0, i);
				}
			}
			
			out.println(toClose.size());
			
			for(int i=0; i<toBuild.size(); ++i) {
				out.print(toClose.get(i).u+" "+toClose.get(i).v+" ");
				out.print(toBuild.get(i).u+" "+toBuild.get(i).v);
				out.println();
			}
			
			
		}
		out.close();	
	}
	
	static class edge{
		int u, v;
		edge(int g, int h){
			u=g; v=h;
		}
	}
	
	static class dsu{
		
		int parent[];
		
		dsu(int v){
			parent=new int[v]; 
		}
		
		void make(int v) {
			parent[v]=v; 
		}
		
		int find(int v) {
			if(parent[v]==v) return v;
			return parent[v]=find(parent[v]);
		}
		
		void union(int u, int v) {
			u=find(u); v=find(v);
			if(u==v) return;
			parent[u]=v;
		}
		
	}
	
	static void sort(int [] a) {
		List<Integer> l=new ArrayList<>();
		for(int i: a) l.add(i);
		Collections.sort(l);
		for(int i=0; i<a.length; i++) a[i]=l.get(i);
	}
	
	static class Reader{
		BufferedReader br;
		StringTokenizer to;
		Reader(InputStream stream){
			br=new BufferedReader(new InputStreamReader(stream));
			to=new StringTokenizer("");
		}
		String next() {
			while(!to.hasMoreTokens()) {
				try {
					to=new StringTokenizer(br.readLine());	
				}catch(IOException e) {}
				
			}
			return to.nextToken();
		}
		int nextInt() {
			return Integer.parseInt(next());
		}
		long nextLong() {
			return Long.parseLong(next());
		}
		
		int [] readArray(int n) {
			int a[]=new int[n];
			for(int i=0; i<n; i++) a[i]=nextInt();
			return a;
		}
	}	
	
}