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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.StringTokenizer;
public class Roads_not_only_in_Berland {
	static HashSet<Integer>reps=new HashSet<>();
	 public static void main(String[]args) throws NumberFormatException, IOException{
		 	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));	
			PrintWriter o=new PrintWriter(System.out);
			int n= Integer.parseInt(in.readLine());
			ArrayList<pair>close=new ArrayList<>();
			DSU d= new DSU(n+1);
			StringTokenizer st = new StringTokenizer(in.readLine());
			int x= Integer.parseInt(st.nextToken());
			int y= Integer.parseInt(st.nextToken());
			d.union(x, y);
			for(int i=1;i<n-1;i++){
				 st = new StringTokenizer(in.readLine());
				 x= Integer.parseInt(st.nextToken());
				 y= Integer.parseInt(st.nextToken());
				 if(d.find(x)!=d.find(y))
				 	d.union(x, y); 
				 else
					close.add(new pair(x, y));
			}
			 for(int i=1;i<n+1;i++){
				 reps.add(d.find(i));
			 }
			 ArrayList<Integer>r=new ArrayList<>();
			 for(int j:reps)
				 r.add(j);
			o.println(close.size());
			if(close.size()!=0)
				for(int i=0;i<close.size();i++)
					o.println(close.get(i).x+" "+close.get(i).y+" "+r.get(i)+" "+r.get(i+1));
			o.flush();
			o.close();
	 }
	 static class DSU{
		 int[]p;
		 int[]s;
		 public DSU(int n){
			p=new int[n]; 
			s=new int[n];
			 for(int i=1;i<n;i++){
				 p[i]=i;
				 s[i]=1;
			 }
		 }
		 public int find(int x){
			 return p[x]=(p[x]==x)?x:find(p[x]);
		 }
		 public int union(int l,int r){
			 	l = find(l);
				r = find(r);
				if (l != r){
					if(s[l]<s[r]){
						p[l] = r;
						return s[r]+=s[l];
					}
					p[r] = l;
					return s[l]+=s[r];
				}
				return s[l];
				
		 }
	 }
	 static class pair{
		 int x,y;
		 public pair(int a,int b){
			 x=a;y=b;
		 }
	 }
}
