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 UnionFind {
  int n;
  vector<int> dad, size;
  UnionFind(int N) : n(N), dad(N), size(N, 1) {
    while (N--) dad[N] = N;
  }
  int find(int u) {
    if (dad[u] == u) return u;
    return dad[u] = find(dad[u]);
  }
  void join(int u, int v) {
    int Ru = find(u), Rv = find(v);
    if (Ru == Rv) return;
    if (size[Ru] > size[Rv]) swap(Ru, Rv);
    --n, dad[Ru] = Rv;
    size[Rv] += size[Ru];
  }
  bool same(int u, int v) { return find(u) == find(v); }
  int get_size(int u) { return size[find(u)]; }
  int number_of_sets() { return n; }
};
const int MAXN = 1e3 + 5;
int n;
bool valid[MAXN];
int vis[MAXN];
vector<pair<int, int>> adj[MAXN];
bool dfs(int u, int p, tuple<int, int, int>& edge) {
  vis[u] = 1;
  for (auto e : adj[u]) {
    if (!valid[e.second] || e.first == p) continue;
    if (vis[e.first] == 1) {
      edge = make_tuple(u, e.first, e.second);
      return true;
    } else if (vis[e.first] == 2)
      continue;
    else if (dfs(e.first, u, edge))
      return true;
  }
  vis[u] = 2;
  return false;
}
int solve(UnionFind& UF, vector<pair<pair<int, int>, pair<int, int>>>& answer) {
  fill(valid, valid + n, true);
  int ans = UF.number_of_sets();
  while (UF.number_of_sets() > 1) {
    fill(vis, vis + n, 0);
    tuple<int, int, int> edge;
    for (int i = 0; i < n; i++) {
      if (vis[i]) continue;
      if (dfs(i, -1, edge)) break;
    }
    int u = UF.find(0);
    int v = u;
    for (int i = 0; i < n; i++) {
      v = UF.find(i);
      if (v != u) break;
    }
    UF.join(u, v);
    valid[get<2>(edge)] = false;
    answer.emplace_back(make_pair(get<0>(edge), get<1>(edge)), make_pair(u, v));
  }
  return ans - 1;
}
int main() {
  ;
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cin >> n;
  UnionFind UF(n);
  int _id = 0;
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    UF.join(u, v);
    adj[u].emplace_back(v, _id);
    adj[v].emplace_back(u, _id);
    _id++;
  }
  vector<pair<pair<int, int>, pair<int, int>>> v;
  cout << solve(UF, v) << '\n';
  for (auto i : v)
    cout << i.first.first + 1 << ' ' << i.first.second + 1 << ' '
         << i.second.first + 1 << ' ' << i.second.second + 1 << '\n';
  return 0;
}
