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 PrI {
	static int[] parent, rank, size;

	public void makeset(int n) {
		rank = new int[n + 1];
		parent = new int[n + 1];
		size = new int[n + 1];
		for (int i = 1; i <= n; i++) {
			parent[i] = i;
			size[i] = 1;
		}
	}

	public int find(int i) {
		if (parent[i] != i)
			parent[i] = find(parent[i]);
		return parent[i];
	}

	public void union(int x, int y) {
		int x_root = find(x);
		int y_root = find(y);
		if (x_root != y_root) {
			if (rank[x_root] < rank[y_root]) {
				parent[x_root] = y_root;
				size[y_root] = size[y_root] + size[x_root];
			} else if (rank[x_root] > rank[y_root]) {
				parent[y_root] = x_root;
				size[x_root] = size[x_root] + size[y_root];
			} else {
				parent[y_root] = x_root;
				size[x_root] = size[x_root] + size[y_root];
				rank[x_root]++;
			}
		}
	}

	public static void main(String[] args) {
		PrI set = new PrI();
		Scanner input = new Scanner(System.in);
		int n = input.nextInt();
		set.makeset(n);
		int[][]closed=new int[n][2];
		int[][]built=new int[n][2];
		int t = 0;
		for (int i = 0; i < n-1; i++) {
			int x = input.nextInt();
			int y = input.nextInt();
			if(set.find(x)==set.find(y)){
				closed[t][0]=x;
				closed[t][1]=y;
				t++;
			}
			else{
				set.union(x,y);
			}
		}
		input.close();
		int counter=0;
		for (int i = 1; i <n-1; i++) {
			for (int j = i+1; j <n; j++) {
				if(set.find(i)!=set.find(j)){
					built[counter][0]=i;
					built[counter][1]=j;
					counter++;
				}
				if (counter==t){
                    break;
				}
			}
			
		}
		System.out.println(t);
		for (int i = 0; i < t; i++) {
			System.out.print(closed[i][0]+" ");
			System.out.print(closed[i][1]+" ");
			System.out.print(built[i][0]+" ");
			System.out.println(built[i][1]);
		}
	}

}
