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.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Stack;
import java.util.StringTokenizer;

public class A {
	
	public static void main(String[] args) throws IOException 
	{
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);

		int n = sc.nextInt();
		DSU dsu = new DSU(n);
		ArrayList<Integer> x = new ArrayList<>(), y = new ArrayList<>();
		for(int i = 1; i < n; ++i)
		{
			int u = sc.nextInt() - 1, v = sc.nextInt() - 1;
			if(!dsu.union(u, v))
			{
				x.add(u);
				y.add(v);
			}
		}
		out.println(x.size());
		Stack<Integer> s = dsu.getComps();
		for(int i = 0; i < x.size(); ++i)
		{
			int u = dsu.find(x.get(i));
			while(dsu.find(s.peek()) == u)
				s.pop();
			out.printf("%d %d %d %d\n", x.get(i) + 1, y.get(i) + 1, x.get(i) + 1, s.peek() + 1);
			dsu.union(s.peek(), u);
		}
		out.flush();
		out.close();
	}
	
	static class DSU
	{
		int[] r, p;
		
		DSU(int n)
		{
			r = new int[n];
			p = new int[n];
			for(int i = 0; i < n; ++i)
				p[i] = i;
		}
		
		int find(int x) { return p[x] == x ? x : (p[x] = find(p[x])); }
		
		boolean union(int x, int y)
		{
			x = find(x);
			y = find(y);
			
			if(x == y)
				return false;
			if(r[x] > r[y])
				p[y] = x;
			else
			{
				p[x] = y;
				if(r[x] == r[y])
					++r[y];
			}
			return true;
		}
		
		Stack<Integer> getComps()
		{
			Stack<Integer> s = new Stack<>();
			for(int i = 0; i < p.length; ++i)
				if(p[i] == i)
					s.push(i);
			return s;
		}
	}

	static class Scanner 
	{
		StringTokenizer st;	
		BufferedReader br;

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

		public Scanner(FileReader r){	br = new BufferedReader(r);}

		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();}


	}
}