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 
/*package whatever //do not write package name here */

import java.io.*;
import java.util.*;
import static java.lang.System.out;

public class GFG {

	static int parent[];
	static int ranks[];


	static void initialize()
	{
	    for(int i=0; i<parent.length; i++)
	    {
	        parent[i]=i;
	    }
	}

	static int find(int x)
	{
	   if(parent[x]==x)
            return x;
    
        parent[x] = find(parent[x]);
    
        return parent[x];
	    
	}


	static void union(int x, int y)
	{
	    int x_rep = find(x);
	    int y_rep = find(y);
	    
	    if(x_rep==y_rep)
	        return;
	    
	    if(ranks[x_rep]<ranks[y_rep])
	        parent[x_rep] = y_rep;
	    
	    else if(ranks[y_rep]<ranks[x_rep])
	        parent[y_rep]=x_rep;
	    
	    else
	        {
	            parent[y_rep] = x_rep;
	            
	            ranks[x_rep]++;
	        }
	    
	    
	   
	}

	public static void main (String[] args) {
		Scanner sc  = new Scanner(System.in);
		ArrayList<Integer> extra = new ArrayList<Integer>();
		
		int n = sc.nextInt();

		parent = new int[n+1];
		ranks = new int[n+1];

		initialize();
		
		
		for(int i=0;i<n-1;i++){
		   	int u = sc.nextInt();
		   	int v = sc.nextInt();
		   	
		   	if(find(u)==find(v)){
		   	    extra.add(u);
		   	    extra.add(v);
		   	}else{
		   	    union(u,v);
		   	}
		   	
		}

	   int min_days = extra.size() / 2;
	   
	   HashSet<Integer> yet_to_connect = new HashSet<Integer>();
	   
	   for(int i=1;i<=n;i++) yet_to_connect.add(find(i));
	   
	   ArrayList<Integer> yet_to_join = new ArrayList<Integer>(yet_to_connect);

		out.print(min_days);
		
		for(int i=0;i<min_days;i++){
		    out.println();
		    out.print(extra.get(0)+" "+extra.get(1)+" ");
		    extra.remove(0);
		    extra.remove(0);
		     out.print(yet_to_join.get(0)+" "+yet_to_join.get(1));
		    yet_to_join.remove(0);
		}
		
	}
}