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>
struct union_find {
  struct node {
    int root, rank, size;
    node(int id = 0) : root(id), rank(0), size(1) {}
  };
  mutable std::vector<node> data;
  union_find(int SZ = 0) : data(SZ) { iota(data.begin(), data.end(), 0); }
  int find(int i) const {
    if (i != data[i].root) data[i].root = find(data[i].root);
    return data[i].root;
  }
  bool is_root(int i) const { return i == find(i); }
  node& root_node(int i) const { return data[find(i)]; }
  bool unite(int a, int b) {
    a = find(a), b = find(b);
    if (a == b) return false;
    if (data[a].rank < data[b].rank) std::swap(a, b);
    data[b].root = a;
    data[a].size += data[b].size;
    if (data[a].rank == data[b].rank) data[a].rank++;
    return true;
  }
  friend void pr(const union_find& u) {
    std::cout << "{";
    bool first = 1;
    for (int i = 0; i < int(u.data.size()); i++) {
      if (u.is_root(i)) {
        if (!first)
          std::cout << ", ";
        else
          first = 0;
        std::cout << "[ " << i << " | rank=" << u.data[i].rank
                  << " size=" << u.data[i].size << " ]";
      }
    }
    std::cout << "}";
  }
};
using namespace std;
int main() {
  int N;
  scanf("%d", &N);
  union_find uf(N);
  std::vector<pair<int, int>> redundant;
  for (int i = 0; i < N - 1; i++) {
    int u, v;
    scanf("%d %d", &u, &v);
    --u, --v;
    if (!uf.unite(u, v)) redundant.emplace_back(u, v);
  }
  std::vector<int> component_roots;
  for (int i = 0; i < N; i++)
    if (uf.is_root(i)) component_roots.push_back(i);
  assert(redundant.size() + 1 == component_roots.size());
  printf("%d\n", redundant.size());
  for (int i = 0; i < redundant.size(); i++) {
    printf("%d %d %d %d\n", redundant[i].first + 1, redundant[i].second + 1,
           component_roots[i] + 1, component_roots[i + 1] + 1);
  }
  return 0;
}
