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.awt.List;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.io.Reader;
import java.io.StreamTokenizer;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.StringTokenizer;
import java.util.Vector;


public class trip {

	/**
	 * @param args
	 * @throws IOException 
	 * @throws NumberFormatException 
	 */
	
	
	
	class Pair{
		public Pair(int x, int v) {
			// TODO Auto-generated constructor stub
			first = x;
			second = v;
		}

		int first, second;
		
		@Override
		public String toString() {
			// TODO Auto-generated method stub
			return first + " " + second + "";
		}
	}
	
	class SNM{
		private ArrayList<Integer> a;
		
		public SNM(int n){
			a = new ArrayList<Integer>(n);
			for (int i = 0; i < n; ++ i)
				a.add(new Integer(i));
		}
		
		public int p(int n){
			if (a.get(n) == n)
				return n;
			a.set(n, p(a.get(n)));
			return a.get(n);
		}
		
		public void Union(int first, int second) {
			if (Math.random() % 2 == 0)
				a.set(p(first), p(second));
			else
				a.set(p(second), p(first));
		}
		
		public boolean one(int first, int second){
			return p(first) == p(second);
		}
	}
	
	ArrayList<Boolean> was;
	ArrayList<Boolean> used;
	ArrayList<Integer> g[];
	
	void dfs(int x, int p, ArrayList<Pair> a){
		if (used.get(x))
			return;
		used.set(x, new Boolean(true));
		
		for (Integer v : g[x]){
			if (v.intValue() != p && used.get(v) && !was.get(v))
				a.add(new Pair(x, v));
			else
				dfs(v, x, a);
		}
		was.set(x, new Boolean(true));
	}
	
	private void solve() throws IOException{
		
		int n = nextInt();
		
		g = new ArrayList[n];
		
		for (int i = 0; i < n; ++ i)
			g[i] = new ArrayList<Integer>();
			
		SNM s = new SNM(n);
		for (int i = 0; i < n - 1; ++ i){
			int l = nextInt() - 1;
			int r = nextInt() - 1;
			g[l].add(r);
			g[r].add(l);
			s.Union(l, r);
		}
		
		ArrayList<Pair> a = new ArrayList<Pair>();
		
		was = new ArrayList<Boolean>();
		used = new ArrayList<Boolean>();
		
		for (int i = 0; i < n; ++ i){
			was.add(new Boolean(false));
			used.add(new Boolean(false));
		}
		
		for (int i = 0; i < n; ++ i){
			if (!was.get(i))
				dfs(i, -1, a);
		}
		
		out.print(a.size() + "\n");
		//out.print(a.toString());
		
		
		for (Pair v : a){
			for (int i = 1; i < n; ++ i){
				if (!s.one(0, i)){
					s.Union(0, i);
					out.print((v.first + 1) + " " + (v.second + 1) + " 1 " + (i + 1) + "\n" );
					break;
				}
			}
		}
		
	}
	
	
	
	StringTokenizer in;
	BufferedReader reader;
	PrintWriter out;
	
	double nextDouble() throws IOException{
		return Double.parseDouble(nextTokinazer());
	}
	
	long nextLong() throws  IOException{
		return Long.parseLong(nextTokinazer());
	}
	
	int nextInt() throws  IOException{
		return Integer.parseInt(nextTokinazer());
	}
	
	private String nextTokinazer() throws IOException{
		while (in == null || !in.hasMoreElements())
			in = new StringTokenizer(reader.readLine());
		return in.nextToken();
	}
	
	void run() throws IOException{
		in = null;
		reader = new BufferedReader(new InputStreamReader(System.in));
		out = new PrintWriter(System.out);
		
		solve();
		out.flush();
	}
	
	
	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub

		new trip().run();
	}

}
