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 RoadsnotonlyinBerland {

	static BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
	static BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(System.out));

	static int n;
	static int[] p, rank;
	
	static void make_set() {
		p = new int[n + 1];
		rank = new int[n + 1];
		for(int i = 1; i <= n; i++)
			p[i] = i;
	}
	
	static int findSet(int i) {
		return p[i] == i ? i : findSet(p[i]);
	}
	
	static boolean isSameSet(int i, int j) {
		return findSet(i) == findSet(j);
	}
	
	static boolean union(int i, int j) {
		if(!isSameSet(i, j)) {
			int x = findSet(i), y = findSet(j);
			if(rank[x] < rank[y]) {
				p[x] = y;
			} else {
				p[y] = x;
			}
			if(rank[x] == rank[y]) rank[x]++;
			return true;
		} else {
			return false;
		}
	}
	
	static class Pair {
		int x;
		int y;
		public Pair(int x, int y) {
			super();
			this.x = x;
			this.y = y;
		}
		
		public boolean equals(Object obj) {
    		if(obj == null || obj.getClass()!= this.getClass()) return false;
            Pair p = (Pair) obj;
            return (this.x==p.x && this.y==p.y);
        }
    	public int hashCode() {
            return Objects.hash(x,y);
        }
	}
	
	static void process() throws Exception {
		make_set();
		List<Pair> redundant_roads = new LinkedList<>();
		String[] data;
		for (int i = 0; i < n-1; i++) {
			data = reader.readLine().split(" ");
			int u = Integer.parseInt(data[0]), v = Integer.parseInt(data[1]);
			if (!union(u, v)) {
				redundant_roads.add(new Pair(u, v));
			}
		}
		List<Pair> new_roads = new LinkedList<>();
		for (int i = 1; i < n; i++) {
			if (!isSameSet(i, i+1)) {
				new_roads.add(new Pair(i, i+1));
				union(i, i+1);
			}
		}
		if (new_roads.size() > redundant_roads.size() || new_roads.size() == 0) {
			writer.write("0");
		} else {
			writer.write(String.format("%d", new_roads.size()));
			writer.newLine();
			for (int i = 0; i < new_roads.size(); i++) {
				Pair n = new_roads.get(i);
				Pair d = redundant_roads.get(i);
				writer.write(String.format("%d %d %d %d", d.x, d.y, n.x, n.y));
			}
		}
		writer.newLine();
	}

	public static void main(String[] args) throws Exception {
		//		writer = new BufferedWriter(new FileWriter("output.txt"));
		n = Integer.parseInt(reader.readLine());
		process();
		writer.close();
		reader.close();
	}

}
