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 

/**
 *
 * @author Daipayan Mukherjee
 * @Jadavpur University,Information Technology
 *
 */
import java.util.*;
import java.io.*;

public class cf25D {
    
    static class Pair{
        int f;
        int s;
        Pair(int f,int s){
            this.f = f;
            this.s = s;
        }
    }

    public static int[] parent;
    public static int[] size;
    
    public static void make_set(int n){
        parent[n] = n;
        size[n] = 1;
    }
    public static int find_set(int n){
        if(parent[n] == n)return n;
        return parent[n] = find_set(parent[n]);
    }
    public static void make_union(int a,int b){
        if(a == b || a == parent[b] || b == parent[a])return;
        
        a = find_set(a);
        b = find_set(b);
        
        if(size[a]<size[b]){
            parent[a] = b;
            size[b] += size[a];
        }else{
            parent[b] = a;
            size[a] += size[b];
        }
    }
    
    
    public static void main(String[] args)throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        
        int n = Integer.parseInt(br.readLine());
        parent = new int[n];
        size = new int[n];
        
        for(int i=0;i<n;i++)
            make_set(i);
    
        List<Pair> toDel = new ArrayList<>();
        
        for(int i=0;i<n-1;i++){
            String[] s = br.readLine().split(" ");
            int first = Integer.parseInt(s[0])-1;
            int second = Integer.parseInt(s[1])-1;
            
            
            if(find_set(first) == find_set(second))
                toDel.add(new Pair(first,second));
            
            make_union(first,second);
        }
        Set<Integer> temp = new HashSet<>();

        for(int i=0;i<n;i++){
            temp.add(parent[i]);
        }
        
        System.out.println(temp.size()-1);
        List<Integer> components = new ArrayList<>(temp);
        int itr = 0;
        int delItr = 0;
        
        while(itr < components.size()-1 && delItr<toDel.size()){
            int comp1 = components.get(itr);
            int comp2 = components.get(itr+1);
            
            make_union(comp1,comp2);

            
            ++comp1;
            ++comp2;
            
            System.out.print((toDel.get(delItr).f+1)+" "+(toDel.get(delItr).s+1));
            ++delItr;
            
                        
            System.out.println(" "+comp1+" "+comp2);
            ++itr;
        }
    }
    
}
