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>
using namespace std;
int pre[1005];
int p[1005];
int t[1005];
int r1[1005];
int r2[1005];
int find(int x) {
  if (x != pre[x])
    return find(pre[x]);
  else
    return x;
}
int main() {
  int i, j, n, x, y, ans, num = 0, d1, d2;
  scanf("%d", &n);
  memset(p, 0, sizeof(p));
  for (i = 1; i <= n; i++) {
    pre[i] = i;
  }
  for (i = 1; i < n; i++) {
    scanf("%d%d", &x, &y);
    r1[num] = x;
    r2[num] = y;
    if (x > y) swap(x, y);
    d1 = find(pre[x]);
    d2 = find(pre[y]);
    if (d1 != x) {
      pre[y] = d1;
    } else if (d2 != y) {
      pre[x] = d2;
    } else
      pre[y] = d1;
    if (d1 == d2) {
      num++;
    }
  }
  ans = 0;
  for (i = 1; i <= n; i++) {
    if (pre[i] == i) {
      t[ans] = i;
      ans++;
    }
  }
  printf("%d\n", num);
  for (i = 0; i < num; i++) {
    printf("%d %d ", r1[i], r2[i]);
    printf("%d %d\n", t[i], t[i + 1]);
  }
  return 0;
}
