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.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.StringTokenizer;

public class Main {
	public static int n, gnum = 0;
	
	public static List<Integer> g[],  gs[];
	public static HashSet<Integer> ex[];
	public static boolean visited[];
	public static void main(String[] arg) {
		FastScanner scan = new FastScanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		n = scan.nextInt();
		g = new ArrayList[n+1];
		ex = new HashSet[n+1];
		gs = new ArrayList[n+1];
		for(int i = 0; i <= n; i++){
			g[i] = new ArrayList<Integer>();
			ex[i] = new HashSet<Integer>();
			gs[i] = new ArrayList<Integer>();
		}
		
		for(int i = 1; i < n; i++){
			int a = scan.nextInt();
			int b = scan.nextInt();
			
			g[a].add(b);
			g[b].add(a);
		}
		
		visited = new boolean[n+1];
		int cnt = 0;
		while(cnt < n){
			for(int i = 1; i <= n; i++)
				if(!visited[i]){
					cnt += dfs(-1, i, gnum++);
					break;
				}
		}
		out.println(gnum - 1);
		int ctarget = 1;
		for(int i = 0; i < gnum; i++){
			for(int c : ex[i]){
				int from = c / 1001;
				int to = c % 1001;
				out.println(from + " " + to + " " + gs[0].get(0) + " " + gs[ctarget++].get(0));
				
			}
		}
		out.close();
	}
	
	public static int dfs(int parent, int node, int group){
		int ret = 1;
		visited[node] = true;
		gs[group].add(node);
		for(int next : g[node]){
			if(visited[next]) {
				if(next != parent) {
					if(!(ex[group].contains(node * 1001 +next) || ex[group].contains(next * 1001 +node)))
						ex[group].add(node * 1001 + next);
				}
				continue;
			}
			ret += dfs(node, next, group);
		}
		return ret;
	}
	
	static class FastScanner {
		BufferedReader br;
		StringTokenizer st;

		FastScanner(InputStream is) {
			try {
				br = new BufferedReader(new InputStreamReader(is));
			} catch (Exception e) {
				e.printStackTrace();
			}
		}

		String next() {
			while (st == null || !st.hasMoreTokens()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (Exception e) {
					return null;
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.valueOf(next());
		}
	}
}