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 
//package algoEx;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class RoadsnotonlyinBerland {
//25D 5.25
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();//city
		int[][] arr = new int[n+1][n+1];
		for(int i=1;i<n;i++){
			int from = sc.nextInt();
			int to = sc.nextInt();
			arr[from][to]++;
			arr[to][from]++;
		}
		//n���� ����,n-1���� ���ΰ� �����Ƿ� ��ü ���ð� ����Ǿ����� �ʴٸ� ������ �����ִ�. 
		//> ������ �ϳ��� ��� ����������� ������ �ϳ��� ����
		boolean[] b = new boolean[n+1];
		List<Integer> list = new ArrayList<>();	
		dfs(arr,b,1);
		for(int i=1;i<=n;i++){
			if(b[i]==false){//
				list.add(i);
				dfs(arr,b,i);
			}
		}
		if(list.size()==0){
			System.out.println("0");
		}else{
			System.out.println(list.size());
			for(int i=0;i<list.size();i++){
				int dfrom = list.get(i)-1;
				int dto = list.get(i)-2;
				int nfrom = dfrom;
				int nto = list.get(i);
				System.out.println(dfrom+" "+dto+" "+nfrom+" "+nto+" ");
			}
		}
		
	}
	static void dfs(int[][] arr, boolean[] b, int x){
		b[x]=true;
		for(int i=1;i<arr.length;i++){
				if(arr[x][i]==1 && b[i]==false){
					dfs(arr,b,i);
				}
		}
	}

}
