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.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class RoadsNotOnkyinBerland {

	public static void main(String[] args) throws IOException {
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		DSU dsu=new DSU(n);
		int []u=new int [n];
		int []v=new int[n];
		
		int ans=0;
		ArrayList<Integer> cyclicEdges=new ArrayList();
		for(int i=0;i<n-1;i++)
		{
			u[i]=sc.nextInt()-1;
			v[i]=sc.nextInt()-1;
			if(dsu.isSameSet(u[i],v[i]))
			{
				ans++;
				cyclicEdges.add(i);
			}
			dsu.union(u[i], v[i]);
		}
		System.out.println(ans);
		int k=0;
		for(int i=0;i<n;i++)
			for(int j=i+1;j<n;j++)
			{
				if(!dsu.isSameSet(i,j))
				{
					System.out.printf("%d %d %d %d\n",u[k]+1,v[k]+1,i+1,j+1);
					k++;
					dsu.union(i, j);
				}
			}
		
		
			
		
		

	}
	static class DSU
	{
		int [] p,r;
		int numSets;
		public DSU(int n)
		{
			p=new int [n];
			r=new int [n];
			for(int i=0;i<n;i++)
				p[i]=i;
			numSets=n;
		}
		public boolean isSameSet(int i,int j)
		{
			return findSet(i)==findSet(j);
		}
		public int findSet(int i)
		{
			if(p[i]==i)
				return i;
			else
				return p[i]=findSet(p[i]);
		}
		public void union(int i,int j)
		{
			
			int x=findSet(i);
			int y=findSet(j);
			if(x==y)
				return ;
			
			numSets--;
			if(r[x]>r[y])
				p[y]=x;
			else
			{
				p[x]=y;
				if(r[x]==r[y])
					r[y]++;
			}
		}
	}
	static class Scanner 
	{
		StringTokenizer st;
		BufferedReader br;

		public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}

		public String next() throws IOException 
		{
			while (st == null || !st.hasMoreTokens()) 
				st = new StringTokenizer(br.readLine());
			return st.nextToken();
		}

		public int nextInt() throws IOException {return Integer.parseInt(next());}
		
		public long nextLong() throws IOException {return Long.parseLong(next());}

		public String nextLine() throws IOException {return br.readLine();}
		
		public double nextDouble() throws IOException
		{
			return Double.parseDouble(next());
		}
		
		public boolean ready() throws IOException {return br.ready();}


	}
}
