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 A {
	public static void main(String[] args)throws Throwable {
		MyScanner sc=new MyScanner();
		PrintWriter pw=new PrintWriter(System.out);
		
		int n=sc.nextInt();
		DSU dsu=new DSU(n);
		ArrayList<String> closed=new ArrayList<String>();
		for(int i=0;i<n-1;i++){
			int u=sc.nextInt();
			int v=sc.nextInt();
			if(!dsu.union(u-1, v-1)){
				closed.add(u+" "+v);
			}
		}
		ArrayList<Integer> sets=new ArrayList<Integer>();
		for(int i=0;i<n;i++)
			if(dsu.findSet(i)==i)
				sets.add(i+1);
		pw.println(closed.size());
		for(int i=0;i<closed.size();i++)
			pw.println(closed.get(i)+" "+sets.get(i)+" "+sets.get(i+1));
		
		pw.flush();
		pw.close();
	}
	static class DSU {
		int n,cntSets;
		int [] rank,parent,setSize;
		
		public DSU(int n) {
			cntSets=n;
			this.n=n;
			rank=new int [n];
			parent=new int [n];
			setSize=new int [n];
			for(int i=0;i<n;i++){
				setSize[i]=1;
				parent[i]=i;
			}
		}
		
		public int findSet(int i){
			return parent[i]==i? i : (parent[i]=findSet(parent[i]));
		}
		
		public boolean union(int u,int v){
			int x=findSet(u);
			int y=findSet(v);
			if(x==y)
				return false;
			if(rank[x]<rank[y]){
				setSize[y]+=setSize[x];
				parent[x]=y;
			}else{
				setSize[x]+=setSize[y];
				if(rank[x]==rank[y])
					rank[x]++;
				parent[y]=x;
			}
			cntSets--;
			return true;
		}
		
	}

	
	static class MyScanner {
		BufferedReader br;
		StringTokenizer st;
		public MyScanner() {
			try {
				br = new BufferedReader(new FileReader(new File("alost.in")));
			} catch (FileNotFoundException e) {
				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 nextInt() {return Integer.parseInt(next());}
		long nextLong() {return Long.parseLong(next());}
		double nextDouble() {return Double.parseDouble(next());}
		String nextLine(){String str = "";
		try {str = br.readLine();}
		catch (IOException e) {e.printStackTrace();}
		return str;}
	}
}