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.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.StringTokenizer;

public class Solution implements Runnable {
	private BufferedReader br;
	private StringTokenizer tok;
	private PrintWriter out;
	
	public static final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
	
	static class Edge {
		int u, v;
		
		Edge(int u, int v) {
			this.u = u;
			this.v = v;
		}
	}
	
	static class DSU {
		int[] parent;
		int[] rnk;
		
		void makeSet(int v) {
			parent[v] = v;
		}
		
		int findSet(int v) {
			if (v == parent[v])
				return v;
			return parent[v] = findSet(parent[v]);
		}
		
		boolean uniteSets(int u, int v) {
			u = findSet(u);
			v = findSet(v);
			if (u != v) {
				if (rnk[u] < rnk[v]) {
					int tmp = u;
					u = v;
					v = tmp;
				}
				parent[v] = u;
				if (rnk[u] == rnk[v]) {
					++rnk[u];
				}
				return true;
			}
			return false;
		}
		
		DSU(int n) {
			parent = new int[n];
			rnk = new int[n];
			for (int i = 0; i < n; ++i) {
				makeSet(i);
			}
		}
	}
	
	void solve() throws IOException {
		int n = nextInt();
		Edge[] edges = new Edge[n - 1];
		for (int i = 0; i + 1 < n; ++i) {
			int u = nextInt();
			int v = nextInt();
			edges[i] = new Edge(u - 1, v - 1);
		}
		DSU myDSU = new DSU(n);
		ArrayList<Edge> badEdges = new ArrayList<Edge>();
		for (int i = 0; i + 1 < n; ++i) 
			if (!myDSU.uniteSets(edges[i].u, edges[i].v)) {
				badEdges.add(edges[i]);
			}
		out.println(badEdges.size());
		HashSet<Integer> check = new HashSet<Integer>();
		ArrayList<Integer> vertexes = new ArrayList<Integer>();
		for (int i = 0; i < n; ++i) 
			if (!check.contains(myDSU.findSet(i))) {
				check.add(myDSU.findSet(i));
				vertexes.add(myDSU.findSet(i));
			}
		for (int i = 0; i < badEdges.size(); ++i) {
			out.println((badEdges.get(i).u + 1) + " " + (badEdges.get(i).v + 1) + " " + (vertexes.get(i) + 1) + " " + (vertexes.get(i + 1) + 1));
		}

	}
		
	public void run() {
		try {
			if (ONLINE_JUDGE) {
				br = new BufferedReader(new InputStreamReader(System.in));
				out = new PrintWriter(System.out);
			} else {
				br = new BufferedReader(new FileReader(new File("input.txt")));
				out = new PrintWriter(new File("output.txt"));
			}
			solve();
			br.close();
			out.close();
		} catch (IOException e) {
			e.printStackTrace();
			System.exit(1);
		}
	}
	
	public static void main(String[] args) {
		new Solution().run();
	}	
	
	String nextToken() throws IOException {
		while (tok == null || !tok.hasMoreTokens())
			tok = new StringTokenizer(br.readLine());
		return tok.nextToken();
	}
	
	String nextString() throws IOException {
		return nextToken();
	}
	
	int nextInt() throws IOException {
		return Integer.parseInt(nextToken());
	}
	
	long nextLong() throws IOException {
		return Long.parseLong(nextToken());
	}
	
	double nextDouble() throws IOException {
		return Double.parseDouble(nextToken());
	}
	
	BigInteger nextBigInteger() throws IOException {
		return new BigInteger(nextToken());
	}
}