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 FastReader sc=null;

	public static void main(String[] args) {
		PrintWriter out=new PrintWriter(System.out);
		sc=new FastReader();
		int n=sc.nextInt();
		DU du=new DU(n);
		ArrayList<Integer> left=new ArrayList<>(),right=new ArrayList<>();
		for(int i=0;i+1<n;i++) {
			int v=sc.nextInt()-1,w=sc.nextInt()-1;
			if(du.connected(du.find(v), du.find(w))) {
				left.add(v);
				right.add(w);
			}
			du.unify(v, w);
		}
		out.println(du.comp-1);
		for(int i=0;i<left.size();i++) {
			int v=left.get(i),w=0,u=right.get(i);
			while(true) {
				if(du.connected(du.find(v), du.find(w)))w++;
				else break;
			}
			v++;w++;u++;
			out.println(v+" "+u+" "+v+" "+w);
		}
		out.close();
		
		
	}
	static class DU{
		 int id[];
		 int size;
		 int comp;
		 int sz[];
		 Map<Integer,ArrayList<Integer>> map=new HashMap<>();
		
		 DU(int size){
			this.size=size;
			id=new int[size];
			sz=new int[size];
			for(int i=0;i<size;i++) {
				id[i]=i;
				sz[i]=1;
			}
			comp=size;
				
		}
		public int find(int p) {
			int root=p;
			
			while(id[root]!=root)
				root=id[root];
			
			while(p!=root) {
				
				int next=id[p];
				id[p]=root;
				p=next;
			}
			return root;
			
		}
		public boolean connected(int p,int q) {
			return id[p]==id[q];
		}
		public int compsize(int p) {
			return sz[find(p)];
		}
		public void unify(int p,int q) {
			int r1=find(p),r2=find(q);
			
			
			if(r1==r2)return;
			
			if(sz[r1]<sz[r2]) {
				sz[r2]+=sz[r1];
				id[r1]=id[r2];
			}
			else {
				sz[r1]+=sz[r2];
				id[r2]=id[r1];
			}
			
			comp--;	
		}	
	}

		
		
	static int[] ruffleSort(int a[]) {
		ArrayList<Integer> al=new ArrayList<>();
		for(int i:a)al.add(i);
		Collections.sort(al);
		for(int i=0;i<a.length;i++)a[i]=al.get(i);
		return a;
	}
	
	
	
	static void print(ArrayList<Integer> al) {
		for(int e:al) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	
	static void print(int a[]) {
		for(int e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	
	static void print(char a[]) {
		for(char e:a) {
			System.out.print(e);
		}
		System.out.println();
	}
	
	static void print(long a[]) {
		for(long e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	
	static int gcd(int a,int b) {
		if(b==0)return a;
		else return gcd(b,a%b);
	}
	
	static class FastReader 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
        int[] readArray(int n) {
    		int a[]=new int [n];
    		for(int i=0;i<n;i++) {
    			a[i]=sc.nextInt();
    		}
    		return a;
    	}
    } 
}






