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.*;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.StringTokenizer;

public class D25 {
	public static void main(String[] args) throws IOException, InterruptedException {
		Scanner sc = new Scanner(System.in);
		PrintWriter pw = new PrintWriter(System.out);
		ArrayList<Point> removable = new ArrayList<>();
		int n = sc.nextInt();
		DSU dsu = new DSU(n + 1);
		for (int i = 0; i < n - 1; i++) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			if (dsu.findSet(a) == dsu.findSet(b))
				removable.add(new Point(a, b));
			else
				dsu.union(a, b);
		}
		pw.println(removable.size());
		HashSet<Integer> set = new HashSet<>();
		ArrayList<Integer> buildable = new ArrayList<>();
		for (int i = 1; i <= n; i++)
			set.add(dsu.findSet(i));
		for (int a : set)
			buildable.add(a);
		for (int i = 0; i < removable.size(); i++)
			pw.println(removable.get(i).x + " " + removable.get(i).y + " " + buildable.get(i) + " "
					+ buildable.get(i + 1));
		pw.flush();
	}

	static class Point {
		int x, y;

		Point(int a, int b) {
			x = a;
			y = b;
		}
	}

	static class DSU {
		int p[];
		int size[];

		DSU(int n) {
			p = new int[n];
			size = new int[n];
			for (int i = 1; i < n; i++) {
				p[i] = i;
				size[i] = 1;
			}
		}

		int findSet(int x) {
			return x == p[x] ? x : (p[x] = findSet(p[x]));
		}

		void union(int a, int b) {
			int x = findSet(a);
			int y = findSet(b);
			if (x == y)
				return;
			if (size[x] >= size[y]) {
				p[y] = x;
				size[x] += size[y];
			} else {
				p[x] = y;
				size[y] += size[x];
			}
		}
	}

	static class Scanner {
		StringTokenizer st;
		BufferedReader br;

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

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

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

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

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

		public char nextChar() throws IOException {
			return next().charAt(0);
		}

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

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

		public void waitForInput() throws InterruptedException {
			Thread.sleep(4000);
		}
	}
}
