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.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
    
    
    public static void main(String args[]){
        Scanner s = new Scanner(System.in);
        
        int n = s.nextInt();
        
        set = new int[n];
        for (int i = 0; i < n; i++) {
            set[i] = i;
        }
        
        ArrayList<String> remove = new ArrayList<String>();
        ArrayList<String> add = new ArrayList<String>();
        
        g = new int[n][n];
        for (int i = 0; i < n - 1; i++) {
            int a = s.nextInt() - 1;
            int b = s.nextInt() - 1;

            int parent1 = findParent(a);
            int parent2 = findParent(b);
            if(parent1 == parent2)
                remove.add((a+1)+" "+(b+1));
            else {
                g[a][b] = g[b][a] = 1;
                set[parent1] = parent2;
            }
        }
        
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                int parent1 = findParent(i);
                int parent2 = findParent(j);
                if(parent1 != parent2){
                    add.add((i+1)+" "+(j+1));
                    set[parent1] = parent2;
                }
            }
        }
        
        System.out.println(remove.size());
        
        for (int i = 0; i < remove.size(); i++) {
            System.out.println(remove.get(i)+" "+add.get(i));
        }
    }
    
    
    static int set[];
    
    static int findParent(int i){
        while(i != set[i]) i = set[i];
        return i;
    }

    static int g[][];
    
    
}
