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.util.*;
import java.io.*;
import java.util.AbstractMap.*;

public class D25 {
	
	public static void main(String args[]) throws Exception{
		FastReader reader = new FastReader();
		int n = reader.I();
		ArrayList<Integer> graph[] = new ArrayList[n];
		for(int i=0;i<n;i++) {
			graph[i] = new ArrayList<>();
		}
		for(int i=0;i<n-1;i++) {
			int from = reader.I() - 1;
			int dest = reader.I() - 1;
			graph[from].add(dest);
			graph[dest].add(from);
		}
		
		int nChanges = 0;
		int changes[][] = new int[n][];
		for(int i=0;i<n;i++) {
			changes[i] = new int[4];
		}
		boolean found = false;
		do{
			found = false;
			int visited[] = new int[n];
			// 0 not visited
			// 1 visiting
			// 2 visited
			int color = 2;
			for(int i=0;i<n;i++) {
				if(found) break;
				if(visited[i] == 0) {
					SimpleEntry<Integer, Integer> toDelete = visit(i, graph, -1, visited, color);
					if(toDelete != null) {
						found = true;
						for(int k=0;k<n;k++) {
							if(visited[k] != visited[i]) {
								changes[nChanges][0] = toDelete.getKey();
								changes[nChanges][1] = toDelete.getValue();
								changes[nChanges][2] = toDelete.getKey();
								changes[nChanges][3] = k;
								nChanges++;
								graph[toDelete.getKey()].remove(toDelete.getValue());
								graph[toDelete.getValue()].remove(toDelete.getKey());
								graph[toDelete.getKey()].add(k);
								graph[k].add(toDelete.getKey());
								break;
							}
						}
					
					}
					color++;
				}
			}
			if(color == 3) break;
		} while(found == true);
		System.out.println(nChanges);
		for(int i=0;i<nChanges;i++) {
			for(int k=0;k<4;k++)
				System.out.print((changes[i][k]+1) + " ");
			System.out.println("");
		}
	}
	
	public static SimpleEntry<Integer, Integer> visit(int now, ArrayList<Integer> graph[], int parent, int visited[], int color) {
		visited[now] = 1;
		SimpleEntry<Integer, Integer> result = null;
		
		for(int i=0;i<graph[now].size();i++) {
			int to = graph[now].get(i);
			if(to == parent) {
				continue;
			}
			if(visited[to] == 1) {
				result = new SimpleEntry<>(now, to);
			}
			if(visited[to] == 0 && result == null) {
				result = visit(to, graph, now, visited, color);
			} else if(visited[to] == 0) {
				visit(to, graph, now, visited, color);
			}
		}
		
		visited[now] = color;
		return result;
	}

	static BufferedReader br;
    static long M = (long) 1e9 + 7;

    static class FastReader {

        StringTokenizer st;

        public FastReader() throws FileNotFoundException {
            //br=new BufferedReader(new FileReader("C:\\Users\\shree\\Downloads\\B-small-practice.in"));
            br = new BufferedReader(new InputStreamReader(System.in));

        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int I() {
            return Integer.parseInt(next());
        }

        long L() {
            return Long.parseLong(next());
        }

        double D() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

        public boolean hasNext() throws IOException {
            while (st == null || !st.hasMoreTokens()) {
                String s = br.readLine();
                if (s == null) {
                    return false;
                }
                st = new StringTokenizer(s);
            }
            return true;
        }

    }
}
