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.*;
import java.io.*;

public class RoadsNotOnlyInBerland {
	static int parent[];
	static int [][]close;
	static int [][] build;

	public static int find(int x) {
		if (parent[x] < 0) {
			return x;
		} else
			return find(parent[x]);
	}

	

	public static void union(int x, int y) {
		int a = find(x);
		int b = find(y);
			parent[a] += parent[b];
			parent[b] = a;
	}

	public static void main(String[] args) throws IOException {
		Scanner sc = new Scanner(System.in);
		PrintWriter pw = new PrintWriter(System.out);
		int x = sc.nextInt();
		parent = new int[x + 1];
		int y = x - 1;
		close = new int[x][2];
		build = new int[x][2];
		Arrays.fill(parent, -1);
		parent[0]=0;
		int cnt=0;
		while (y-- > 0) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			if (find(a) == find(b)) {
				close[cnt][0]=a;
				close[cnt][1]=b;
				cnt++;
			} else {
				union(a, b);
			}
		}
		int c=0;
		for (int i = 2; i <= x; i++) {
			if (find(i) != find(1)) {
				union(i, 1);
				build[c][0]=i;
				build[c++][1]=1;
			}
		}
		
		pw.println(cnt);
		for (int i = 0; i < cnt; i++) {
			
			pw.println(close[i][0] + " " + close[i][1] + " " + build[i][0] + " " + build[i][1]);
		}
		pw.close();
	}

	static class Scanner {
		BufferedReader br;
		StringTokenizer st;

		public Scanner(InputStream s) {
			br = new BufferedReader(new InputStreamReader(s));
		}

		public String next() throws IOException {
			while (st == null || !st.hasMoreTokens())
				st = new StringTokenizer(br.readLine());
			return st.nextToken();
		}

		public int nextInt() throws IOException {
			return Integer.parseInt(next());
		}

		public long nextLong() throws IOException {
			return Long.parseLong(next());
		}

		public double nextDouble() throws IOException {
			return Double.parseDouble(next());
		}

		public int[] nextIntArr(int n) throws IOException {
			int[] arr = new int[n];
			for (int i = 0; i < n; i++) {
				arr[i] = Integer.parseInt(next());
			}
			return arr;
		}

		public String nextLine() throws IOException {
			return br.readLine();
		}

		public boolean ready() throws IOException {
			return br.ready();
		}
	}
}
