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.StringTokenizer;
import java.util.TreeSet;

public class P1 {

	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int n = sc.nextInt();
		DS uf = new DS(n);
		for(int i = 0; i < n - 1; i++)
			uf.merge(sc.nextInt() - 1, sc.nextInt() - 1);
		
		for (int i = 0; i < n; i++)
			uf.close();
		
		TreeSet<Integer> reps = new TreeSet<>();
		for (int i = 0; i < uf.p.length; i++)
			reps.add(uf.p[i]);
		
		out.println(uf.wastedX.size());
		for (int i = 0; i < uf.wastedX.size(); i++)
		{
			out.println((uf.wastedX.get(i) + 1) + " " + (uf.wastedY.get(i) + 1) + " " + (reps.pollFirst() + 1) + " " + (reps.first() + 1));
		}
		out.flush();
		out.close();
	}
	
	
	static class DS {
		ArrayList<Integer> wastedX = new ArrayList<>();
		ArrayList<Integer> wastedY = new ArrayList<>();
		int sz;
		int [] p, rank;
		DS (int size) {sz = size; rank = new int[size]; p = new int[size]; for (int i = 0; i < sz; i++) p[i] = i;}
		int find (int idx) {return p[idx] == idx ? idx : (p[idx] = find(p[idx]));}
		void merge (int i, int j) {
			int x = find(i), y = find(j);
			if (x == y){
				wastedX.add(i);
				wastedY.add(j);
				return;
			}
			
			if (rank[x] > rank[y])
				p[y] = x;
			else 
			{
				p[x] = y;
				if (rank[x] == rank[y]) rank[y]++;
			}
		}
		
		void close ()
		{
			for (int i = 0; i < sz; i++)
				find(i);
		}
	}
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	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 {
			String x = next();
			StringBuilder sb = new StringBuilder("0");
			double res = 0, f = 1;
			boolean dec = false, neg = false;
			int start = 0;
			if (x.charAt(0) == '-') {
				neg = true;
				start++;
			}
			for (int i = start; i < x.length(); i++)
				if (x.charAt(i) == '.') {
					res = Long.parseLong(sb.toString());
					sb = new StringBuilder("0");
					dec = true;
				} else {
					sb.append(x.charAt(i));
					if (dec)
						f *= 10;
				}
			res += Long.parseLong(sb.toString()) / f;
			return res * (neg ? -1 : 1);
		}

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

		public boolean nextEmpty() throws IOException {
			String s = nextLine();
			st = new StringTokenizer(s);
			return s.isEmpty();
		}
	}
}