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 codechef; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Codechef
{
    static int p[];
	static ArrayList<Integer> del;
 
	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st;
 
		int n = Integer.parseInt(in.readLine());
		p = new int[n];
		for (int i = 0; i < p.length; i++) {
			p[i] = i;
		}
 
		del = new ArrayList<Integer>();
 
		for (int i = 0; i < n - 1; i++) {
			st = new StringTokenizer(in.readLine());
			int a = Integer.parseInt(st.nextToken()) - 1;
			int b = Integer.parseInt(st.nextToken()) - 1;
			union(a, b);
		}
 
		System.out.println(del.size() / 2);
		for (int k = 0; k < del.size(); k += 2) {
			int i = del.get(k);
			int j = del.get(k + 1);
			int u = parent(i);
			int v = -1;
			for (int l = 0; l < n; l++) {
				if (parent(l) != u) {
					v = l;
					break;
				}
			}
			union(u, v);
			System.out.printf("%d %d %d %d\n", i + 1, j + 1, u + 1, v + 1);
		}
 
	}
 
	static void union(int i, int j) {
		int pi = parent(i);
		int pj = parent(j);
 
		if (pi == pj) {
			del.add(i);
			del.add(j);
		} else {
			p[pi] = pj;
		}
	}
 
	static int parent(int i) {
		if (p[i] != i) {
			p[i] = parent(p[i]);
			return p[i];
		}
		return i;
	}
}
