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.StringTokenizer;

public class RoadsNotOnlyInBerland {
	public static int parent[],rank[]; 
	public static int find_set (int v) 
	{
		if (v == parent[v])
			return v;
		return parent[v] = find_set (parent[v]);
	}
	public static void union_sets (int a, int b) 
	{
		 a = find_set (a);
		 b = find_set (b);
		 if (a != b) 
		 {
			if (rank[a] < rank[b])
			{
				int temp = a;
				a = b;
				b = temp;
			}
			parent[b] = a;
			if (rank[a] == rank[b])
				++rank[a];
		 }
	}
	public static void main(String args[]) throws IOException
	{
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		parent = new int [n];
		rank = new int[n];
		for (int i=0;i<n;i++)
			parent[i] = i;
		ArrayList<Integer> roadFrom=new ArrayList<Integer>();
		ArrayList<Integer> roadTo=new ArrayList<Integer>();
		for (int i=1;i<n;i++)
		{
			int a = sc.nextInt()-1;
			int b = sc.nextInt()-1;
			if (find_set(a)==find_set(b))
			{
				roadFrom.add(a+1);
				roadTo.add(b+1);
			}
			else
				union_sets(a, b);
		}
		System.out.println(roadFrom.size());
		int pointer = 0;
		for (int i=1;i<n;i++)
		{
			if (find_set(0)==find_set(i))
				continue;
			union_sets(0,i);
			System.out.println(roadFrom.get(pointer)+" "+roadTo.get(pointer)+" 1 "+(i+1));
			pointer++;
		}
	}
	static class Scanner 
	{
		StringTokenizer st;
		BufferedReader br;
	
		public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}
	
		public String next() throws IOException 
		{
			while (st == null || !st.hasMoreTokens()) 
				st = new StringTokenizer(br.readLine());
			return st.nextToken();
		}
	
		public int nextInt() throws IOException {return Integer.parseInt(next());}
		
		public long nextLong() throws IOException {return Long.parseLong(next());}
	
		public String nextLine() throws IOException {return br.readLine();}
		
		public boolean ready() throws IOException {return br.ready();}
	
	
	}
}
