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 
#include <bits/stdc++.h>
const int maxn = 1001;
int pa[maxn];
struct Node {
  int u, v;
} cut[maxn];
int finding(int x) {
  if (x != pa[x]) {
    pa[x] = finding(pa[x]);
  }
  return pa[x];
}
void unio(int a, int b) {
  int x = finding(a);
  int y = finding(b);
  pa[x] = y;
}
int main() {
  int n, ans = 0;
  scanf("%d", &n);
  for (int i = 0; i <= n; i++) {
    pa[i] = i;
  }
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    scanf("%d %d", &a, &b);
    if (finding(a) == finding(b)) {
      cut[ans].u = a;
      cut[ans].v = b;
      ans++;
    } else {
      unio(a, b);
    }
  }
  printf("%d\n", ans);
  int num = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (finding(i) != finding(j)) {
        printf("%d %d %d %d\n", cut[num].u, cut[num].v, i, j);
        unio(i, j);
        num++;
        if (num == ans) break;
      }
    }
  }
}
