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 {
	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter pw = new PrintWriter(System.out);
		int n = Integer.parseInt(br.readLine());
		DSU d = new DSU(n);
		ArrayList<Pair> remove = new ArrayList<Pair>();
		ArrayList<Pair> build = new ArrayList<Pair>();
		ArrayList<Integer> representative = new ArrayList<Integer>();
		TreeSet<Integer> tree = new TreeSet<Integer>();
		for (int i = 0; i < n-1; i++) {
			StringTokenizer st = new StringTokenizer(br.readLine());
			int u = Integer.parseInt(st.nextToken())-1;
			int v = Integer.parseInt(st.nextToken())-1;
			if(d.isSameSet(u, v))
			{
				remove.add(new Pair(u, v));
			}
			else
			{
				d.union(u, v);
			}
		}
		for (int i = 0; i < n; i++) {
			if(!tree.contains(d.findSet(i)))
			{
				representative.add(d.findSet(i));
				tree.add(d.findSet(i));
			}
		}
		for (int i = 0; i < representative.size(); i++) {
			for (int j = i+1; j < representative.size(); j++) {
				build.add(new Pair(representative.get(i), representative.get(j)));
			}
		}
		pw.println(remove.size());
		for (int i = 0; i < remove.size(); i++) {
			pw.println((remove.get(i).x+1) + " " + (remove.get(i).y+1) + " " + (build.get(i).x+1) + " " + (build.get(i).y+1));
		}
		pw.close();
	}
	
	static class DSU
	{
		int[] parent, size;
		
		public DSU(int n)
		{
			parent = new int[n];
			size = new int[n];
			for (int i = 0; i < parent.length; i++) {
				parent[i]=i;
				size[i]=1;
			}
		}
		
		public int findSet(int i)
		{
			if(parent[i]==i)return i;
			return parent[i]=findSet(parent[i]);
		}
		
		public boolean isSameSet(int i, int j)
		{
			return findSet(i)==findSet(j);
		}
		
		public void union(int i, int j)
		{
			if(isSameSet(i, j))return;
			int I = findSet(i);
			int J = findSet(j);
			if(size[I]<=size[J])
			{
				size[J]+=size[I];
				parent[I]=parent[J];
			}
			else
			{
				size[I]+=size[J];
				parent[J]=parent[I];
			}
		}
	}
	static class Pair
	{
		int x, y;
		public Pair(int x, int y)
		{
			this.x=x; this.y=y;
		}
	}
}
