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 Berland {

	public int[] Parent;
	public int[] counter;
	public int[][] old;
	public int[][] New;
	int cnt = 0;

	void merge(int a, int b) {
		int ParentA = find(a);
		int ParentB = find(b);

		if (ParentA == ParentB) {
			old[cnt][0] = a;
			old[cnt][1] = b;
			cnt++;
			return;

		}

		if (counter[a] >= counter[b]) {
			Parent[ParentB] = ParentA;
			counter[a] += counter[b];
		} else {
			Parent[ParentA] = ParentB;
			counter[b] += counter[a];
		}
	}

	int find(int a) {
		if (a == Parent[a])
			return a;

		return Parent[a] = find(Parent[a]);
	}

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		Berland br = new Berland();
		Scanner Scan = new Scanner(System.in);
		int n = Scan.nextInt();

		br.counter = new int[n + 1];
		br.Parent = new int[n + 1];

		br.old = new int[n][2];
		br.New = new int[n][2];

		for (int i = 1; i <= n; i++)
			br.Parent[i] = i;

		for (int i = 1; i <= n; i++)
			br.counter[i] = 1;

		for (int i = 0; i < n-1 ; i++)
			br.merge(Scan.nextInt(), Scan.nextInt());

		int j = 0;
		for (int i = 1; i <= n; i++) {
			if (br.find(i) != br.find(br.old[1][0])) {
				br.New[j][0] = br.old[0][0];
				br.New[j][1] = i;
			}

		}

		System.out.println(br.cnt);
		for (int i = 0; i < br.cnt; i++) {
			System.out.print(br.old[i][0] + " ");
			System.out.print(br.old[i][1] + " ");
			System.out.print(br.New[i][0] + " ");
			System.out.println(br.New[i][1]);

		}

	}

}
