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 A {
	
	static int[] par,size;
	static int count=0;
	public static void main(String[] args)throws IOException 
	{	
		
		FastReader f=new FastReader();
		StringBuffer sb = new StringBuffer();
		
		int n=f.nextInt();
		int m=n-1;
		
		par=new int[n+1];
		size=new int[n+1];
		
		for(int i=1;i<=n;i++)
		{
			par[i]=i;
			size[i]=1;
		}
		
		List<String> del=new ArrayList<>();
		int ans=0;
		for(int i=0;i<m;i++)
		{
			int x=f.nextInt();
			int y=f.nextInt();
//			System.out.println("x,y = "+x+","+y);
			count=0;
			union(x,y);
			
			if(count>0)
			{	
				ans++;
				del.add(x+" "+y);
			}
//			System.out.print("parent --> ");
//			for(int j=1;j<=n;j++)
//				System.out.print(par[j]+" ");
//			System.out.println();
//			System.out.print("size --> ");
//			for(int j=1;j<=n;j++)
//				System.out.print(size[j]+" ");
//			System.out.println("\n");
		}
		List<Integer> leader=new ArrayList<>();
		for(int i=1;i<=n;i++)
		{
			if(par[i]==i)
				leader.add(i);	
		}
		sb.append(ans+"\n");
//		System.out.println("leader = "+leader);
		int one=leader.get(0);
		for(int i=1;i<leader.size();i++)
		{
			int two=leader.get(i);
			sb.append(del.get(i-1)+" "+(one+" "+two)+"\n");
			one=two;
		}
		System.out.println(sb);
	}
	//path compression 
	static int find(int n) {
		while (true) {
			if (par[n] == n)
				return n;
			n = par[par[n]];
		}
	}

	//union by rank
	static void union(int a, int b) 
	{	
		
		a = find(a);
		b = find(b);

		if (a == b)
		{	
			count++;
			return;
		}
		if (size[a] < size[b]) {
			int temp = a;
			a = b;
			b = temp;
		}
		par[b] = a;
		size[a] += size[b];
		size[b]=0;
	}
}
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; 
    } 
} 