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.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import java.util.SortedMap;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeMap;
import java.util.TreeSet;

  public class Test
 {
      static class DisSet
      {
          int countSets;
          int[] rank;
          int[] parent;
          int[] setSize;
          
          DisSet(int n)
          {
              countSets = n;
              rank = new int[n+1];
              parent = new int[n+1];
              setSize = new int[n+1];
              for(int i = 1 ; i<=n ; i++)
              {
                  rank[i] = 0;
                  parent[i] = i;
                  setSize[i] = 1;
              }
          }
          
          boolean isSameSet(int i , int j)
          {
              return find(i) == find(j);
          }
          
          int find(int x)
          {
              if(parent[x] == x)
                  return x;
              return parent[x] = find(parent[x]);
          }
          
          void union(int i , int j)
          {
              if(isSameSet(i,j))
              {
                  q.add(new pair(i,j));
                  return;
              }
              
              int a = find(i);
              int b = find(j);
              int r1 = rank[a];
              int r2 = rank[b];
              this.countSets--;
              if(r1>r2)
              {
                  parent[b] = a;
                  setSize[a] += setSize[b];
              }
              else if(r2>r1)
              {
                  parent[a] = b;
                  setSize[b] += setSize[a];
              }
              else
              {
                   parent[b] = a;
                  setSize[a] += setSize[b];
                  rank[a]++;
              }
          }
          
          ArrayList<Integer> getRoots()
          {
              ArrayList<Integer> list = new ArrayList(this.countSets);
              int z =  parent.length;
              for(int i = 1 ; i<z; i++)
                  if(parent[i] ==  i)
                      list.add(i);
              
              return list;
          }
      }
      
      static class pair{
          int a ,  b;
          pair(int x , int y){
              a = x; b = y;
          }
      }
              
    
    static PrintWriter pw = new PrintWriter(System.out);
    static Queue<pair> q = new LinkedList();

    public static void main(String[] args)throws Exception
    {
        Reader.init(System.in);
        int n = Reader.nextInt();
        StringBuilder res = new StringBuilder();
        DisSet dis = new DisSet(n);
        
        for(int i = 1 ; i<n ; i++)
        {
            int a = Reader.nextInt();
            int b = Reader.nextInt();
            dis.union(a, b);
        }

        
        ArrayList<Integer> roots = dis.getRoots();
        int zz = 0;
        res.append(dis.countSets - 1).append("\n");
        while(!q.isEmpty())
        {
            pair p = q.poll();
            res.append(p.a).append(" ").append(p.b).append(" ").append(roots.get(zz)).append(" ").append(roots.get(zz+1)).append("\n");
            zz++;
        }
        
        if(dis.countSets > 1)
            pw.print(res);
        else
            pw.print(0);
        pw.close();
    }
    
}


class Reader {

    static BufferedReader reader;
    static StringTokenizer tokenizer;

    public static int pars(String x) {
        int num = 0;
        int i = 0;
        if (x.charAt(0) == '-') {
            i = 1;
        }
        for (; i < x.length(); i++) {
            num = num * 10 + (x.charAt(i) - '0');
        }

        if (x.charAt(0) == '-') {
            return -num;
        }

        return num;
    }

    static void init(InputStream input) {
        reader = new BufferedReader(
                new InputStreamReader(input));
        tokenizer = new StringTokenizer("");
    }

    static void init(FileReader input) {
        reader = new BufferedReader(input);
        tokenizer = new StringTokenizer("");
    }

    static String next() throws IOException {
        while (!tokenizer.hasMoreTokens()) {
            tokenizer = new StringTokenizer(
                    reader.readLine());
        }
        return tokenizer.nextToken();
    }

    static int nextInt() throws IOException {
        return pars(next());
    }

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

    static double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }
}