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.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class RoadsNotOnlyInBerland {

	static class UnionFind {
		int[] rep, rank;

		UnionFind(int n) {
			rep = new int[n];
			rank = new int[n];
			for (int i = 0; i < n; i++)
				rep[i] = i;
		}

		boolean union(int x, int y) {
			if (isSameSet(x, y))
				return false;
			x = findSet(x);
			y = findSet(y);
			if (rank[x] > rank[y])
				rep[y] = x;
			else {
				rep[x] = y;
				if (rank[x] == rank[y])
					rank[y]++;
			}
			return true;
		}

		int findSet(int x) {
			if (rep[x] == x)
				return x;
			return rep[x] = findSet(rep[x]);
		}

		boolean isSameSet(int x, int y) {
			return findSet(x) == findSet(y);
		}
	}

	public static void main(String[] args) throws IOException {
		MyScanner sc = new MyScanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		int n = sc.nextInt();
		UnionFind uf = new UnionFind(n + 1);
		ArrayList<Integer[]> bad = new ArrayList<>();
		for (int i = 0; i < n - 1; i++) {
			int u = sc.nextInt(), v = sc.nextInt();
			if (!uf.union(u, v))
				bad.add(new Integer[] { u, v });
		}
		out.println(bad.size());
		for (int i = 2, idx = 0; i <= n; i++)
			if (uf.union(1, i)) {
				Integer[] x = bad.get(idx++);
				out.printf("%d %d ", x[0], x[1]);
				out.printf("1 %d\n", i);
			}
		out.flush();
		out.close();
	}

	static class MyScanner {
		StringTokenizer st;
		BufferedReader br;

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

		public MyScanner(String file) throws IOException {
			br = new BufferedReader(new FileReader(new File(file)));
		}

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

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