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.util.*;
import java.io.*;
import java.math.BigDecimal;

public class Task {

	public static void main(String[] args) throws Exception {

		new Task().solve();
	}
	
	
	void solve() throws Exception {
		Reader in = new Reader("1.in");
		PrintWriter out = new PrintWriter( new BufferedWriter( new OutputStreamWriter(System.out) ) );
		
		
		int n = in.nextInt();
		int[][] g = new int[n+1][n+1];
		Queue<Integer> adds = new LinkedList<>();
		Queue<Integer> err = new LinkedList<>();
		DSU dsu = new DSU(n+2);
		
		for (int i = 0; i < n-1; i++) {
			int x = in.nextInt();
			int y = in.nextInt();
			g[x][y] = 1;
			g[y][x] = 1;
			if ( !dsu.incomp(x, y) ) {
				dsu.union(x, y);
			}
			else
			{
				dsu.union(x, y);
				err.add(x);
				err.add(y);
			}
		}
		
		int ans = 0;
		
		
		for (int i = 1; i <= n-1; i++) {
			for (int j = i+1; j <= n; j++) {
				if ( !dsu.incomp(i, j) ) {
					ans++;
					dsu.union(i, j);
					adds.add(i);
					adds.add(j);
				}
			}
		}
		
		
		out.println(ans);
		for (int i = 0; i < ans; i++) {
			out.println(err.poll()+" "+err.poll()+" "+adds.poll()+" "+adds.poll());
		}
		
		out.flush();
		out.close();
		
	}
	
	class Pair implements Comparable<Pair>{
		int i;
		int a;
		Pair(int i, int a) {
			this.i = i;
			this.a = a;
		}
		@Override
		public int compareTo(Pair pair) {
			return a > pair.a ? -1: a < pair.a ? 1: 0;
		}
	}
	
	class DSU {
		
		int size;
		int[] sz;
		int[] set;
		
		DSU(int size) {
			this.size = size;
			set = new int[size+1];
			sz = new int[size+1];
			for(int i = 0; i < size; i++) {
				set[i] = i;
				sz[i] = 1;
			}
		}
		
		int get(int x) {
			if (set[x] == x)
				return x;
			else
				return set[x] = get(set[x]);
		}
		
		void union(int x, int y) {
				x = get(x);
				y = get(y);
				if (sz[x] < sz[y]) {
					set[x] = y;
					sz[y] += sz[x];
				}	
				else
				{	
					set[y] = x;
					sz[x] += sz[y];
				}				

		}
		
		boolean incomp(int x, int y) {
			return get(x) == get(y);
		}
		
		int getSize(int x) {
			x = get(x);
			return sz[x];
		}
		
	}
	
	class Reader {
		
		BufferedReader br;
		StringTokenizer token = null;
		
		Reader(String file) throws Exception {
			br = new BufferedReader( new InputStreamReader(System.in) );
			
		}
		
		String next() throws Exception {
			while (token == null || !token.hasMoreElements()) {
				token = new StringTokenizer(br.readLine());
			}
			return token.nextToken();
		}
		
		int nextInt() throws Exception {
			return Integer.parseInt(next());
		}
		
		long nextLong() throws Exception {
			return Long.parseLong(next());
		}
		
		double nextDouble() throws Exception {
			return Double.parseDouble(next());
		}
		
	}

}
