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.HashSet;
import java.util.Set;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.StringTokenizer;
import java.util.TreeMap;
import java.util.Arrays;
import java.util.HashMap;
public class dsu{
	static int[]p;
	static int[]size;
	static boolean[]v;
	static int findset(int x) {
		if(p[x]==x)return x;
		return p[x]=findset(p[x]);
	}
	static void merge(int x,int y) {
		int a=findset(x);
		int b=findset(y);
		if(a!=b) {
			if(size[a]<size[b]) {
				int tmp=a;
				a=b;
				b=tmp;
			}
			size[a]+=size[b];
			p[b]=a;
			v[b]=false;
		}
		
	}
	public static void main(String[] args) throws IOException {
		MScanner sc=new MScanner(System.in);
		//BufferedReader br=new BufferedReader((new InputStreamReader(System.in)));
		PrintWriter pw=new PrintWriter(System.out);
		int n=sc.nextInt();
		p=new int[n];size=new int[n];v=new boolean[n];
		for(int i=0;i<n;i++) {
			p[i]=i;size[i]=1;v[i]=true;
		}
		ArrayList<Integer>unnec=new ArrayList<Integer>();
		
		for(int i=0;i<n-1;i++) {
			int x=sc.nextInt()-1;int y=sc.nextInt()-1;
			if(findset(x)==findset(y)) {
				unnec.add(x);
				unnec.add(y);
			}
			else
				merge(x,y);
		}
		int cur=-1;int numS=0;
		for(int i=0;i<n;i++) {
			if(v[i] && cur==-1) {
				cur=i;numS++;
			}
			else
				if(v[i]) {
					numS++;
				}
		}
		pw.println(numS-1);
		if(numS>1) {
			for(int i=cur+1;i<n;i++) {
				if(v[i]) {
					pw.println((unnec.remove(0)+1)+" "+(unnec.remove(0)+1)+" "+(cur+1)+" "+(i+1));
					cur=i;
				}
			}
		}
		pw.close();
		pw.flush();
 
	}

	static class MScanner 
	{
	    StringTokenizer st;
	    BufferedReader br;

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

	    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
	    {
	        String x = next();
	        StringBuilder sb = new StringBuilder("0");
	        double res = 0, f = 1;
	        boolean dec = false, neg = false;
	        int start = 0;
	        if(x.charAt(0) == '-')
	        {
	            neg = true;
	            start++;
	        }
	        for(int i = start; i < x.length(); i++)
	            if(x.charAt(i) == '.')
	            {
	                res = Long.parseLong(sb.toString());
	                sb = new StringBuilder("0");
	                dec = true;
	            }
	            else
	            {
	                sb.append(x.charAt(i));
	                if(dec)
	                    f *= 10;
	            }
	        res += Long.parseLong(sb.toString()) / f;
	        return res * (neg?-1:1);
	    }
	    
	    public boolean ready() throws IOException {return br.ready();}


	}
}
 
