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.Scanner;
public class Cthulhu {
	static int[] p;
	static int[] rank;
	static int[][] old;
	static int[] neww;

	static int t=0;
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		p = new int[n+1];
		rank = new int[n+1];
		old= new int[n][2];
		neww= new int[n];

		for(int i = 1 ; i< n+1 ;i++)
		{
			create(i);
		}
		for(int i=1 ; i < n ; i++)
		{
			int one = scan.nextInt();
			int two = scan.nextInt();  
			merge(one , two);
			
		}
		
		System.out.println(t);
		for(int i=1 ; i<=n ; i++){
			
			for(int g=0; g<t ; g++){
			if(find(old[g][0])!=find(i)){
				
				neww[g]=i;
				
			}
			}
		}
		
		for(int i = 0 ; i<t ; i++){
			
			System.out.println(old[i][0]+" " +old[i][1]+" " +old[i][0]+" " +neww[i] );
			
		}


	}
	public static void create(int x){
		p[x]=x;
		rank[x]=0;
	}
	public static void merge(int x , int y){
		int px=find(x);
		int py=find(y);
		if(px == py)
		{
			

			old[t][0]= x;
			old[t][1]= y;
		
			t++;
			
		}
		else if(rank[px] > rank[py]){
			p[py]=px;
		}
		else
			p[px]=py;
		
		if(rank[px]== rank[py])
		{
			rank[py]++;
		}
		
	}
	public static int find(int x){
		
		if(p[x]==x)
			return x;
		
		return p[x]=find(p[x]);
	}

}
