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.math.*;
import java.io.*;
import java.util.*;
public class Main {
	static BufferedReader br;
	static StringTokenizer st;
	static PrintWriter pw;

	public static void main(String[] args) throws IOException {
		br = new BufferedReader(new InputStreamReader(System.in));
		pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
		//int qq = 1;
		int qq = Integer.MAX_VALUE;
		//int qq = readInt();
		for(int casenum = 1; casenum <= qq; casenum++) {
			int n = readInt();
			boolean[][] graph = new boolean[n][n];
			boolean[][] save = new boolean[n][n];
			ArrayList<Integer> root = new ArrayList<Integer>();
			for(int a = 1; a < n; a++)	{
				int x = readInt();
				int y = readInt();
				x--;
				y--;
				graph[Math.min(x,y)][Math.max(x,y)] = true;
			}
			boolean[] seen = new boolean[n];
			for(int i = 0; i < n; i++)	{
				if(seen[i])	{
					continue;
				}
				seen[i] = true;
				LinkedList<Integer> q = new LinkedList<Integer>();
				q.add(i);
				root.add(i);
				while(!q.isEmpty())		{
					int curr = q.removeFirst();
					for(int k = 0; k < graph[curr].length; k++)	{
						if(graph[Math.min(curr,k)][Math.max(curr,k)] && !seen[k])	{
							seen[k] = true;
							q.addLast(k);
							save[Math.min(curr,k)][Math.max(curr,k)] = true;
						}
					}
				}
			}
			ArrayList<State> remove = new ArrayList<State>();
			for(int i = 0; i < n; i++)	{
				for(int j = i+1; j < n; j++)	{
					if(graph[i][j] && !save[i][j])	{
						remove.add(new State(i,j));
					}
				}
			}
			pw.println(remove.size());
			for(int i = 0; i < remove.size(); i++)	{
				pw.printf("%d %d %d %d\n", 1+remove.get(i).a, 1+remove.get(i).b, 1+root.get(i), 1+root.get(i+1));
			}
		}
		pw.close();
	}

	static class State {
		public int a,b;

		public State(int a, int b) {
			super();
			this.a = a;
			this.b = b;
		}
		
	}
	
	public static double readDouble() throws IOException {
		return Double.parseDouble(nextToken());
	}

	public static int readInt() throws IOException {
		return Integer.parseInt(nextToken());
	}

	public static long readLong() throws IOException {
		return Long.parseLong(nextToken());
	}

	public static String nextToken() throws IOException {
		while(st == null || !st.hasMoreTokens())	{
			if(!br.ready())	{
				pw.close();
				System.exit(0);
			}
			st = new StringTokenizer(br.readLine());
		}
		return st.nextToken();
	}
}

