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.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.StringTokenizer;

public class RoadsnotonlyinBerland {
   public static void main(String[] args) throws IOException {
	   Scanner s = new Scanner(System.in);
	   int n = s.nextInt();
	   DSU d = new DSU(n);
	   //boolean f = true;
	   ArrayList<Destroyable> a = new ArrayList<>();
	   HashSet<Integer> hs = new HashSet<>();
	  // int general = 0;
	   while(n-- >1){
		   int x = s.nextInt();
		   int y = s.nextInt();
		   if(d.findRep(x) == d.findRep(y))
			   a.add(new Destroyable(x, y));
		   else{ 
			  int union = d.Union(x, y);
//			   if(f) {
//				   general = union;
//				   f = false;
//			   }
//			   else  hs.add(union);
//			   if(hs.contains(union))
//				   hs.remove(union);
			   
		    }
	   }
	  
	   System.out.println(a.size());
//	   for (int i = 0; i < a.size(); i++) {
//		   System.out.println(a.get(i).x +" "+ a.get(i).y +"whaaaattt");
//	}
	   for (int i = 1; i < d.p.length; i++) {
			hs.add(d.p[i]);
	}
	  
	   if(a.size() >0){
		  ArrayList<Integer> r = new ArrayList<>();
		  for(int v: hs){
			  r.add(v);
		  }
//		  for (int j = 0; j < r.size(); j++) {
//			System.out.println(r.get(i) +" ???");
//		}
//		  for (int i = 0; i < r.size(); i++) {
//			System.out.println(r.get(i)+" whyy");
//		}
	   for (int j = 0; j < a.size(); j++) {
		   System.out.println(a.get(j).x +" "+ a.get(j).y +" "+r.get(j) +" " +r.get(j+1));
	}
		  
		   
	
	
	   }   
	   
	   
	
}
   static class Destroyable{
	   int x;
	   int y;
	   public Destroyable(int s,int t){
		   x =s;
		   y =t;
	   }
   }
   
   static class DSU{
		int[] p;
		int max;
		int[] size;
		public DSU(int n){
			p = new int[n+1];
			size = new int[n+1];
			max = 1;
			for (int i = 1; i < p.length; i++) {
				p[i] = i;
				size[i]=1;
			}
			
		}
		public int findRep(int x){
			return p[x]= (x==p[x])? x:  findRep(p[x]);
		}
		
		public int Union(int x,int y){
			x = findRep(x);
			y = findRep(y);
			if(x==y)
				return 0;
			if(size[x] >= size[y]){
				size[x] += size[y];
				max = Math.max(size[x], max);
				p[y] = x;
				return x;
						
			}
			else {
				size[y] += size[x];
				max = Math.max(size[y], max);
				p[x] = y;
				return y;
			}
			
		}
	}
   
   static class Scanner
 	{
 	    StringTokenizer st;
 	    BufferedReader br;

 	    public Scanner(InputStream system) {br = new BufferedReader(new InputStreamReader(system));}
 	    public String next() throws IOException
 	    {
 	        while (st == null || !st.hasMoreTokens())
 	            st = new StringTokenizer(br.readLine());
 	        return st.nextToken();
 	    }
 	    public String nextLine()throws IOException{return br.readLine();}
 	    public int nextInt() throws IOException {return Integer.parseInt(next());}
 	    public double nextDouble() throws IOException {return Double.parseDouble(next());}
 	    public char nextChar()throws IOException{return next().charAt(0);}
 	    public Long nextLong()throws IOException{return Long.parseLong(next());}
 	    public boolean ready() throws IOException{return br.ready();}
 	    public void waitForInput() throws InterruptedException {Thread.sleep(4000);}
 	}
   
   
   
   
}
