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;
const int N = 1006;
struct Edge {
  int s, t, next;
  bool flag;
  void init(int a, int b, int c) { s = a, t = b, next = c, flag = false; }
} edge[N * 2];
int head[N], tot;
void add_edge(int s, int t) {
  edge[tot].init(s, t, head[s]);
  head[s] = tot++;
  edge[tot].init(t, s, head[t]);
  head[t] = tot++;
}
int n, cnt[N], fa[N];
bool vs[N];
void DFS(int u, int pre) {
  vs[u] = 1;
  for (int e = head[u]; e != -1; e = edge[e].next) {
    int v = edge[e].t;
    if (v == pre || vs[v]) continue;
    edge[e].flag = 1, edge[e ^ 1].flag = 1;
    DFS(v, u);
  }
}
int find_fa(int x) {
  if (fa[x] != x) fa[x] = find_fa(fa[x]);
  return fa[x];
}
int main() {
  scanf("%d", &n);
  memset(head, -1, sizeof head);
  tot = 0;
  for (int i = 1; i <= n; i++) fa[i] = i;
  for (int i = 1, a, b; i < n; i++) {
    scanf("%d%d", &a, &b);
    add_edge(a, b);
    int f1 = find_fa(a);
    int f2 = find_fa(b);
    if (f1 != f2) fa[f1] = f2;
  }
  memset(vs, 0, sizeof vs);
  for (int i = 1; i <= n; i++)
    if (!vs[i]) DFS(i, -1);
  memset(cnt, 0, sizeof cnt);
  int ans = 0;
  for (int i = 0; i < tot; i += 2)
    if (!edge[i].flag) {
      ans++;
      cnt[find_fa(edge[i].s)]++;
      cnt[find_fa(edge[i].t)]++;
    }
  printf("%d\n", ans);
  for (int e = 0; e < tot; e += 2) {
    if (edge[e].flag) continue;
    printf("%d %d ", edge[e].s, edge[e].t);
    for (int i = 1; i <= n; i++) {
      if (cnt[find_fa(i)] != 0 || find_fa(i) == find_fa(edge[e].s)) continue;
      printf("%d %d\n", edge[e].s, i);
      fa[find_fa(i)] = find_fa(edge[e].s);
      break;
    }
    cnt[find_fa(edge[e].s)]--;
    cnt[find_fa(edge[e].t)]--;
  }
  return 0;
}
