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.*;
public class Main{
	static int[] par;
	public static void main(String args[]) throws Exception{
		BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
		int n=Integer.parseInt(in.readLine());
		StringBuilder sb=new StringBuilder("");
			
		par=new int[n];
		for(int i=0;i<n;i++)
			par[i]=i;
			
		ArrayDeque<Edge> pq=new ArrayDeque<Edge>();
		for(int i=0;i<n-1;i++){
			String[] ab=in.readLine().split(" ");
			int a=Integer.parseInt(ab[0])-1;
			int b=Integer.parseInt(ab[1])-1;
			pq.add(new Edge(a, b));
		}
		
		ArrayDeque<Edge> closed=new ArrayDeque<Edge>();
		int time=0;
		while(pq.size()>0){
			Edge curr=pq.poll();
			int par1=find(curr.from);
			int par2=find(curr.to);
			
			if(par1!=par2){
				union(par1, par2);
			} else {
				time++;
				closed.add(curr);
			}
		}
		
		sb.append(time+"\n");
		for(int i=1;i<n;i++){
			int pari1=find(i);
			int pari2=find(i-1);
			
			if(pari1!=pari2){
				Edge cl=closed.poll();
				sb.append((cl.from+1)+" "+(cl.to+1)+" "+(i+1)+" "+i+"\n");
				union(pari1, pari2);
			}
		}
		
		System.out.print(sb);
	}
	
	static int find(int i){
		if(par[i]!=i) par[i]=find(par[i]);
		return par[i];
	}
	
	static void union(int i, int j){
		par[i]=par[j];
	}
	
	static class Edge{
		int w;
		int from, to;
		
		public Edge(int from, int to){
			this.from=from;
			this.to=to;
		}
	}
}