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.util.*;
import java.io.*;

public class C{
	public static int[] parent;
	public static int[] size;
	public static int rep(int a)
	{
		int i = a;
		while(i!=parent[i])i=parent[i];
		while(a!=i)
		{
			int temp = parent[a];
			parent[a] = i;
			a = temp;
		}
		return i;
	}
	
	public static boolean isCycle(int a, int b)
	{
		int aRep = rep(a);
		int bRep = rep(b);
		if(aRep==bRep)return true;
		if(size[aRep]>size[bRep])
		{
			int temp = aRep;
			aRep = bRep;
			bRep = temp;
		}
		parent[aRep] = bRep;
		size[bRep] += size[aRep];
 		return false;
	}
	
	public static void main(String[] args)
	{
		FastScanner fs = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		int n = fs.nextInt();
		parent = new int[n+1];
		size = new int[n+1];
		for(int i=1;i<=n;i++)size[i]=1;
		for(int i=1;i<=n;i++)parent[i]=i;
		List<Integer[]> arr = new ArrayList();
		List<Integer> arr1 = new ArrayList();
		for(int i=0;i<n-1;i++)
		{
			int a = fs.nextInt(); int b = fs.nextInt();
			if(isCycle(a,b)==true)
			{
//				System.out.println("I m here");
				arr.add(new Integer[] {a,b});
			}
		}
		int count = 0;
//		for(int i=1;i<=n;i++)
//		{
//			System.out.print(parent[i]+" ");
//		}
//		System.out.println();
		for(int i=1;i<=n;i++)
		{
			if(parent[i]==i)
			{
				count++;
				arr1.add(i);
			}
		}
		System.out.println(count-1);
		int start = 0;
		for(Integer[] a : arr)
		{
//			System.out.println("I am here");
			System.out.print(a[0]+" "+a[1]+" "+arr1.get(start)+" "+arr1.get(start+1));
			System.out.println();
			start++;
		}
	}

	
	
	
	
	
	static class FastScanner {
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		String next() {
			while (!st.hasMoreTokens())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		
		int nextInt() {
			return Integer.parseInt(next());
		}
		int[] readArray(int n) {
			int[] a=new int[n];
			for (int i=0; i<n; i++) a[i]=nextInt();
			return a;
		}
		long nextLong() {
			return Long.parseLong(next());
		}
	}

	
}
class Person{
	public int id;
	public int m;
	public Person(int id, int m)
	{
		this.id = id;
		this.m = m;
	}
}




