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.*;
import java.io.*;
import java.lang.*;

public class Main {
    

    public static class DisjointSet {
        int a[];

        public DisjointSet(int n) {
            a = new int[n];
            Arrays.fill(a, -1);
        }


        private int root(int i) {
            if(a[i] == -1) {
                return i;
            } else {
                return a[i] = root(a[i]);
            }
        }

        public boolean union(int i, int j) {
            int l = root(i);
            int r = root(j);
            if(l != r) {
                a[l] = r;
                return true;
            }
            return false;
        }
        
        public boolean inOne(int i, int j) {
            return root(i) == root(j);
        }
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        DisjointSet s = new DisjointSet(n);
        int d[][] = new int[n][2];
        int a[][] = new int[n][2];
        int dc = 0;
        int ac = 0;
        for(int i = 0; i < n - 1; i++) {
            int l = in.nextInt();
            int r = in.nextInt();
            if(!s.union(l - 1, r - 1)) {
                d[dc][0] = l;
                d[dc][1] = r;
                dc++;
            }
        }
        
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < n; j++) {
                if(s.union(i, j)) {
                    a[ac][0] = i + 1;
                    a[ac][1] = j + 1;
                    ac++;
                }
            }
        }
        System.out.println(dc);
        for(int i = 0; i < ac; i++) {
            System.out.println(d[i][0] + " " + d[i][1] + " " + a[i][0] + " " + a[i][1]);
        }
    } 
}