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.*;

public class d implements Runnable {

	int[] p, r;

	void init(int n) {
		p = new int[n];
		r = new int[n];
		for (int i = 0; i < n; i++) {
			p[i] = i;
		}
	}

	int get(int i) {
		if (p[i] != i)
			p[i] = get(p[i]);
		return p[i];
	}

	void union(int i, int j) {
		i = get(i);
		j = get(j);
		if (i == j)
			return;
		if (r[i] == r[j]) {
			r[i]++;
		}
		if (r[i] >= r[j]) {
			p[j] = i;
		} else {
			p[i] = j;
		}
	}

	class Pair {
		int a, b;

		public Pair(int a, int b) {
			this.a = a;
			this.b = b;
		}

	}

	private void solve() throws IOException {
		int n = nextInt();
		init(n);
		ArrayList<Pair> remove = new ArrayList<Pair>();
		ArrayList<Pair> add = new ArrayList<Pair>();
		for (int i = 0; i < n - 1; i++) {
			int u = nextInt() - 1;
			int v = nextInt() - 1;
			if (get(u) != get(v))
				union(u, v);
			else {
				remove.add(new Pair(u + 1, v + 1));
			}
		}
		for (int i = 0; i < n; i++) {
			for (int j = i + 1; j < n; j++) {
				if (get(i) != get(j)) {
					add.add(new Pair(i + 1, j + 1));
					union(i, j);
				}
			}
		}
		if (add.size() != remove.size())
			throw new AssertionError();
		out.println(remove.size());
		for (int i = 0; i < remove.size(); i++) {
			out.println(remove.get(i).a + " " + remove.get(i).b + " "
					+ add.get(i).a + " " + add.get(i).b);
		}
	}

	public static void main(String[] args) {
		new Thread(new d()).start();
	}

	BufferedReader br;
	StringTokenizer st;
	PrintWriter out;
	boolean eof = false;

	public void run() {
		Locale.setDefault(Locale.US);
		try {
			br = new BufferedReader(new InputStreamReader(System.in));
			out = new PrintWriter(System.out);
			solve();
			out.close();
		} catch (Throwable e) {
			e.printStackTrace();
			System.exit(239);
		}
	}

	String nextToken() {
		while (st == null || !st.hasMoreTokens()) {
			try {
				st = new StringTokenizer(br.readLine());
			} catch (Exception e) {
				eof = true;
				return "0";
			}
		}
		return st.nextToken();
	}

	int nextInt() {
		return Integer.parseInt(nextToken());
	}

	long nextLong() {
		return Long.parseLong(nextToken());
	}

	double nextDouble() {
		return Double.parseDouble(nextToken());
	}
}