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.io.PrintWriter;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
import java.util.StringTokenizer;




public class Main{
	public static ArrayList<Integer>[] adjlist;
	public static Queue<edge> removable;
	public static void main(String[] args) throws IOException, InterruptedException {
		
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		
		int n = sc.nextInt();
		
		
		removable = new LinkedList<edge>();
		
		UnionFind u = new UnionFind(n);
		
		
		
		for (int i = 0; i < n-1; i++) {
			int src = sc.nextInt()-1;
			int dest = sc.nextInt()-1;
			
			if (!u.isSameSet(src, dest)) 
				u.unionSet(src, dest);
			else {
				removable.add(new edge(src, dest));
			}	
		}
		
		out.println(u.numsets-1);
		for (int i = 1; i < n; i++) {
			if (u.numsets <= 1) 
				break;
			
			if (!u.isSameSet(0, i)) {
				out.println((removable.peek().src+1)+" "+(removable.poll().dest+1)+" "+1+" "+(i+1));
				u.unionSet(0, i);
			}
		}
		
		
		
		out.flush();

	}
	
	static class UnionFind{
		 int[] p,rank,setsize;
		 int numsets;
		 
		 public UnionFind(int i) {
			 p = new int[numsets = i];
			 rank = new int[i];
			 setsize = new int[i];
			 
			 for (int j = 0; j < i; j++) { p[j] = j;setsize[j]= 1; } 
				
		 }
		 
		 public int findSet(int i) {return p[i] == i?i:(p[i] = findSet(p[i]));}
		 
		 public boolean isSameSet(int i,int j) {return findSet(i) == findSet(j);}
		 
		 public void unionSet(int i,int j) {
			 if (isSameSet(i, j)) 
				return;
			 numsets--;
			 int x = findSet(i), y = findSet(j);
			 
			 if (rank[x]> rank[y]) { p[y]= x; setsize[x] += setsize[y];  }
				
			 else {
				p[x] = y; 
				setsize[y] += setsize[x];  
				if (rank[x] == rank[y]) { rank[y]++; }
					
			}
		 }
	}
	
	
static class edge{
	int src;
	int dest;
	
	public edge(int s,int d) {
		src = s;
		dest = d;
	}
}
	
}
class Scanner {
	StringTokenizer st;
	BufferedReader br;

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

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

	public String nextLine() throws IOException {
		return br.readLine();
	}

	public int nextInt() throws IOException {
		return Integer.parseInt(next());
	}

	public double nextDouble() throws IOException {
		return Double.parseDouble(next());
	}

	public char nextChar() throws IOException {
		return next().charAt(0);
	}

	public Long nextLong() throws IOException {
		return Long.parseLong(next());
	}

	public boolean ready() throws IOException {
		return br.ready();
	}

	public void waitForInput() throws InterruptedException {
		Thread.sleep(3000);
	}


}