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.awt.Point;
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.StringTokenizer;

public class C {
	
	public static void main(String[] args) throws Exception
	{
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		int n = sc.nextInt();
		DSU dsu = new DSU(n);
		ArrayList<Point> removable = new ArrayList<>();
		for(int i = 0; i < n-1; i++)
		{
			int u = sc.nextInt()-1, v = sc.nextInt()-1;
			boolean merged = dsu.mergeSets(u, v);
			if(!merged) removable.add(new Point(u, v));
		}
		ArrayList<Integer> roots = new ArrayList<>();
		for(int i = 0; i < n; i++)
			if(dsu.isRoot(i))
				roots.add(i);
		out.println(dsu.numSets-1);
		for(int i = 0; i < roots.size()-1; i++)
		{
			Point r = removable.get(i);
			out.printf("%d %d %d %d\n",r.x + 1, r.y + 1, roots.get(i) + 1, roots.get(i + 1) + 1);
		}
		out.flush();
		out.close();
	}
	
	static class DSU
	{
		int N, numSets;
		int[] p, rank;
		DSU(int n)
		{
			N = n;
			numSets = N;
			p = new int[N]; rank = new int[N];
			for(int i = 0; i < N; i++)
				p[i] = i;
		}
		int getRoot(int x){return p[x] == x? x : getRoot(p[x]);}
		boolean mergeSets(int x, int y)
		{
			x = getRoot(x); y = getRoot(y);
			if(x == y)
				return false;
			if(rank[x] > rank[y])
				p[y] = x;
			else if(rank[x] < rank[y])
				p[x] = y;
			else
			{
				p[y] = x;
				rank[x]++;
			}
			numSets--;
			return true;
		}
		boolean isRoot(int x){return p[x] == x;}
	}
	
	static 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(){for(long i = 0; i < 3e9; i++);}
	}
}