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 Main {

	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		PrintWriter pw = new PrintWriter(System.out);
		int n = sc.nextInt();
		DSU set = new DSU(n);
		TreeMap<Integer, Integer> map = new TreeMap<>();
		for (int i = 1; i < n; i++) {
			int x = sc.nextInt()-1;
			int y = sc.nextInt()-1;
			if(set.isSameSet(x, y)) {
				map.put(x, y);
			} else {
				set.union(x, y);
			}
		}
		int roads = set.size -1;
		pw.println(roads);
		for(Map.Entry<Integer, Integer> curr : map.entrySet()) {
			if(roads == 0) break;
			for (int i = 0; i < n; i++) {
				if(!set.isSameSet(i, curr.getKey())) {
					pw.printf("%d %d %d %d\n", curr.getKey()+1, curr.getValue()+1, curr.getKey()+1, i+1);
					roads--;
					set.union(i, curr.getKey());
				}
			}
		}
		pw.close();
	}

	static class DSU {
		int[] p;
		int[] rank;
		int[] count;
		int size;

		public DSU(int n) {
			p = new int[n];
			for (int i = 0; i < p.length; i++)
				p[i] = i;
			rank = new int[n];
			count = new int[n];
			Arrays.fill(count, 1);
			size = n;
		}

		public void union(int i, int j) {
			int pi = findSet(i);
			int pj = findSet(j);
			if (pi == pj)
				return;
			if (rank[pj] > rank[pi]) {
				p[pi] = pj;
				count[pj] += count[pi];
				count[pi] = 0;
			}
			else {
				p[pj] = pi;
				count[pi] += count[pj];
				count[pj] = 0;
			}
			if (rank[pi] == rank[pj])
				rank[pi]++;
			size--;
		}

		public int findSet(int i) {
			if (p[i] == i)
				return i;
			return p[i] = findSet(p[i]);
		}

		public boolean isSameSet(int i, int j) {
			return findSet(i) == findSet(j);
		}
	}

	static class Scanner {
		StringTokenizer st;
		BufferedReader br;

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

		public Scanner(String file) throws FileNotFoundException {
			br = new BufferedReader(new FileReader(file));
		}

		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();
		}

	}
}
