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;
struct edge {
  int u, v;
} del[4000];
struct data {
  int u, v, x, y;
} lms[4000];
int fa[1013], sum[1013], tot, n, x, y, fx, fy, ans;
inline int find(int x) {
  if (fa[x] == x) return x;
  fa[x] = find(fa[x]);
  return fa[x];
}
inline long long read() {
  long long cnt = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    cnt = cnt * 10 + ch - 48;
    ch = getchar();
  }
  return cnt * f;
}
void load() {
  for (int i = 1; i <= n; i++) fa[i] = i;
}
int main() {
  n = read();
  load();
  for (int i = 1; i < n; i++) {
    x = read();
    y = read();
    fx = find(x);
    fy = find(y);
    if (fx != fy)
      fa[fy] = fx;
    else {
      del[++tot].u = x;
      del[tot].v = y;
    }
  }
  for (int i = 1; i < n; i++)
    for (int j = i + 1; j <= n; j++) {
      fx = find(i);
      fy = find(j);
      if (fx != fy) {
        fa[fy] = fx;
        lms[++ans].u = del[tot].u;
        lms[ans].v = del[tot--].v;
        lms[ans].x = i;
        lms[ans].y = j;
      }
    }
  printf("%d\n", ans);
  for (int i = 1; i <= ans; i++)
    printf("%d %d %d %d\n", lms[i].u, lms[i].v, lms[i].x, lms[i].y);
  return 0;
}
