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.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import java.util.StringTokenizer;

public class D {
        private Map<Long, Node> map = new HashMap<>();
        static int dsu[];
    class Node {
        long data;
        Node parent;
        int rank;
    }

    /**
     * Create a set with only one element.
     */
    public void makeSet(long data) {
        Node node = new Node();
        node.data = data;
        node.parent = node;
        node.rank = 0;
        map.put(data, node);
    }

    /**
     * Combines two sets together to one.
     * Does union by rank
     *
     * @return true if data1 and data2 are in different set before union else false.
     */
    public boolean union(long data1, long data2) {
        Node node1 = map.get(data1);
        Node node2 = map.get(data2);

        Node parent1 = findSet(node1);
        Node parent2 = findSet(node2);

        //if they are part of same set do nothing
        if (parent1.data == parent2.data) {
            return false;
        }

        //else whoever's rank is higher becomes parent of other
        if (parent1.rank >= parent2.rank) {
            //increment rank only if both sets have same rank
            parent1.rank = (parent1.rank == parent2.rank) ? parent1.rank + 1 : parent1.rank;
            parent2.parent = parent1;
        } else {
            parent1.parent = parent2;
        }
        return true;
    }

    /**
     * Finds the representative of this set
     */
    public long findSet(long data) {
        return findSet(map.get(data)).data;
    }

    /**
     * Find the representative recursively and does path
     * compression as well.
     */
    private Node findSet(Node node) {
        Node parent = node.parent;
        if (parent == node) {
            return parent;
        }
        node.parent = findSet(node.parent);
        return node.parent;
    }

    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.flush();
    }
    static class TaskB{
        void solve(int test , InputReader in , PrintWriter out) {
            
            int n = in.nextInt();
            D ds = new D();
            ArrayList<pair> rem = new ArrayList();
            for (int i = 1; i <= n; i++) 
                ds.makeSet(i);
            
            for (int i = 0 , a , b; i < n-1; i++) {
                a = in.nextInt();
                b = in.nextInt();
                if(ds.findSet(a) == ds.findSet(b))
                    rem.add(new pair(a,b));
                else {
//                    ds.findSet(a = ds.findSet(b);
                    ds.union(a, b);
                }
            }
            ArrayList<Integer> c = new ArrayList();
            for (int i = 1; i <= n; i++) {
                if(ds.findSet(i) == i)
                    c.add(i);
            }
            out.println(c.size()-1);
            for (int i = 0; i < c.size()-1; i++) {
                System.out.printf("%d %d %d %d",rem.get(i).first , rem.get(i).second,c.get(i),c.get(i+1));
            }
            }
        }
        public static class pair{
        int first , second;
        pair(int first,int second){
            this.first = first;
            this.second = second;
        }
    }
       static class InputReader{
       public BufferedReader reader;
       public StringTokenizer tk;
       public InputReader(InputStream stream){
           reader = new BufferedReader(new InputStreamReader(stream));
           tk = null;
       }
       String next(){
           while (tk == null || !tk.hasMoreTokens()) {
               try{
                   tk = new StringTokenizer(reader.readLine());
               }
               catch(IOException e){
                   throw new RuntimeException();
               }
           }
           return tk.nextToken();
       }
       int nextInt(){
           return Integer.parseInt(next());
       }
       long nextLong(){
           return Long.parseLong(next());
       }        
   } 
}
