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 static java.lang.Math.*;
import static java.util.Arrays.*;
import java.util.*;
import java.io.*;

public class Main {
	public static void main(String[] args) throws Exception {
		Class<?> here = new Object(){}.getClass().getEnclosingClass();
		try {
			String packageName = here.getPackage().getName();
			packageName = "src/" + packageName.replaceAll("\\.", "/") + "/";
			System.setIn(new FileInputStream(packageName + "input.txt"));
//			System.setOut(new PrintStream(new FileOutputStream(packageName + "output.txt")));
		} catch (FileNotFoundException e) {
		} catch (NullPointerException e) {
		}

		Object o = Class.forName(here.getName()).newInstance();
		o.getClass().getMethod("run").invoke(o);
	}

	static void tr(Object... os) {
		System.err.println(deepToString(os));
	}
	class UnionFind {
		int[] data;

		public UnionFind(int n) {
			data = new int[n];
			fill(data, -1);
		}

		boolean link(int x, int y) {
			x = root(x);
			y = root(y);
			if (x != y) {
				if (data[y] < data[x]) {
					data[y] += data[x];
					data[x] = y;
				} else {
					data[x] += data[y];
					data[y] = x;
				}
			}
			return x != y;
		}

		int root(int x) {
			return data[x] < 0 ? x : (data[x] = root(data[x]));
		}

		int size(int x) { // 要素xが含まれる集合の大きさ
			return -data[root(x)];
		}

		boolean same(int x, int y) { // 同じ集合ならtrue
			return root(x) == root(y);
		}

		int num() { // 異なる集合の数
			int res = 0;
			for (int i = 0; i < data.length; i++)
				res += (root(i) == i) ? 1 : 0;
			return res;
		}
	}

	MyScanner sc = null;
	PrintWriter out = null;
	public void run() throws Exception {
		sc = new MyScanner(System.in);
		out = new PrintWriter(System.out);
		for (;sc.hasNext();) {
			solve();
			out.flush();
		}
		out.close();
	}

	void solve() {
		int n = sc.nextInt();

		UnionFind uf = new UnionFind(1000);
		List<Integer> waste = new LinkedList<Integer>();

		for (int i = 0; i < n - 1; i++) {
			int a = sc.nextInt() - 1;
			int b = sc.nextInt() - 1;
			if (uf.same(a, b)) {
				waste.add(a+1);
				waste.add(b+1);
			} else {
				uf.link(a, b);
			}
		}

		out.println(waste.size() / 2);
		int p = 0;
		for (int i = 1; i < n; i++) if (!uf.same(0, i)) {
			uf.link(0, i);
			out.println(waste.get(p++) + " " + waste.get(p++) + " " + 1 + " " + (i + 1));
		}
	}


	void print(int[] a) {
		out.print(a[0]);
		for (int i = 1; i < a.length; i++) out.print(" " + a[i]);
		out.println();
	}

	class MyScanner {
		String line;
		BufferedReader reader;
		StringTokenizer tokenizer;

		public MyScanner(InputStream stream) {
			reader = new BufferedReader(new InputStreamReader(stream));
			tokenizer = null;
		}

		public void eat() {
			while (tokenizer == null || !tokenizer.hasMoreTokens()) {
				try {
					line = reader.readLine();
					if (line == null) {
						tokenizer = null;
						return;
					}
					tokenizer = new StringTokenizer(line);
				} catch (IOException e) {
					throw new RuntimeException(e);
				}
			}
		}

		public String next() {
			eat();
			return tokenizer.nextToken();
		}

		public String nextLine() {
			try {
				return reader.readLine();
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
		}

		public boolean hasNext() {
			eat();
			return (tokenizer != null && tokenizer.hasMoreElements());
		}

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

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

		public double nextDouble() {
			return Double.parseDouble(next());
		}
		public int[] nextIntArray(int n) {
			int[] a = new int[n];
			for (int i = 0;i < n;i++) a[i] = nextInt();
			return a;
		}
	}
}
