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 static java.lang.Integer.parseInt;
import static java.lang.System.out;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;

public class Main {

	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

		int N = Integer.parseInt(in.readLine().trim());		
		int id[] = new int[N], sz[] = new int[N]; 
		for (int i = 0;i<N;i++) 
		{ id[i] = i; sz[i] = 1; } 

		ArrayList<String> list = new ArrayList<String>();

		int count = 0;
		for (int z = N-2; z>=0; z--)
		{ 	
			String[] arr = in.readLine().trim().split("\\s+");
			int i, j, p = parseInt(arr[0])-1, q = parseInt(arr[1])-1; 
			for (i = p; i != id[i]; i = id[i]) 
				id[i] = id[id[i]]; 
			for (j = q; j != id[j]; j = id[j]) 
				id[j] = id[id[j]]; 

			if (i == j) 
			{
				list.add((p+1)+ " " + (q+1));
				count++;
				continue; 
			}
			if (sz[i] < sz[j]) 
			{ id[i] = j; sz[j] += sz[i]; } 
			else { id[j] = i; sz[i] += sz[j]; } 			
		} 
		
		boolean[] flag = new boolean[N+1];		
		out.println(count);		
		int prev = id[0], index = 0;
		
		int j1 = 0;
		for (; j1 != id[j1]; j1 = id[j1]) 
			id[j1] = id[id[j1]]; 
		flag[j1] = true;
		
		for (int i = 1; i < id.length; i++)
		{
			int j = i;
			for (; j != id[j]; j = id[j]) 
				id[j] = id[id[j]]; 
			
			//id[i]!=prev&&
			if (!flag[j]){
				out.println(list.get(index)+ " " +(i+1)+" "+ (prev+1));
				prev = i;
				index++;
				flag[j] = true;
			}
		}
	}	
}


