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.Scanner;


public class Main {
    public static void main(String[] args) {
        Scanner r = new Scanner(System.in);
        
        int N = r.nextInt();
        
        int[] p = new int[N];
        for(int i = 0; i < N; i++)
            p[i] = i;
        
        ArrayList<int[]> edges = new ArrayList<int[]>();
        ArrayList<int[]> res = new ArrayList<int[]>();
        for(int k = 0; k < N-1; k++){
            int i = r.nextInt()-1;
            int j = r.nextInt()-1;
            
            if(isSame(i, j, p)){
//              int l = 0;
//              for(;l < N; l++)
//                  if(!isSame(i, l, p))break;
//              
//              union(i, l, p);
//              res.add(new int[]{i, j, i, l});
                edges.add(new int[]{i, j});
            }else{
                union(i, j, p);
            }
            
        }
        
        for(int k = 0; k < edges.size(); k++){
            int i = edges.get(k)[0];
            int j = edges.get(k)[1];
            
//          if(isSame(i, j, p)){
            int l = 0;
            for(;l < N; l++)
                if(!isSame(i, l, p))break;
//              
            union(i, l, p);
            res.add(new int[]{i, j, i, l});
//          }else{
//              union(i, j, p);
//          }
        }
        
        System.out.println(res.size());
        for(int[] k : res){
            for(int i = 0; i < 4; i++)
                System.out.print(k[i]+1 + " ");
            System.out.println();
        }
    }
    static int find(int i, int[] p){
        if(p[i] == i)return i;
        else return p[i] = find(p[i], p);
    }
    static void union(int i, int j, int[] p){
        p[find(i, p)] = find(j, p);
    }
    static boolean isSame(int i, int j, int[] p){
        return find(i, p) == find(j, p);
    }
}
