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.InputStreamReader;
import java.util.ArrayList;


public class Roads {
	
	static int[] parent;
	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		parent = new int[n+1];
		int a, b;
		String[] input;
		ArrayList<String> borrar = new ArrayList<String>();
		ArrayList<String> agregar = new ArrayList<String>();
		for (int i = 0; i < n-1; i++){
			input = br.readLine().split(" ");
			a = Integer.parseInt(input[0]);
			b = Integer.parseInt(input[1]);
			if (find(a) == find(b)){ borrar.add(a + " " + b); }
			else { merge(a, b); }
		}
		
		for (int i = 1; i <= n; i++){
			for (int j = 2; j <= n; j++){
				if (find(i) != find(j)){
					agregar.add(i + " " + j);
					merge(i, j);
				}
			}
		}
		int m = borrar.size();
		System.out.println(m);
		for (int k = 0; k < m; k++){
			System.out.println(borrar.get(k) + " " + agregar.get(k));
		}
		
	}
	
	static int find(int x){
		if (parent[x] == 0){ parent[x] = x; }
		int root = x;
		while(parent[root] != root){ root = parent[root]; }
		
//		int old;
//		while(x != root){
//			old = x;
//			x = parent[x];
//			parent[old] = root;
//		}
		
		return root;
	}
	
	static void merge(int a, int b){
//		int x = find(a);
//		int y = find(b);
//		if (x == y) return;
//		if (parent[x] > parent[y]){
//			int aux = x;
//			x = y;
//			y = aux;
//		}
//		parent[x] += parent[y];
//		parent[y] = x;
		parent[b] = find(a);
	}
}
