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;
class DisjointSet {
  int N, ncomp;
  vector<int> par, rank;

 public:
  DisjointSet(size_t _N) : N(_N), ncomp(_N), par(_N, -1), rank(_N, 0) {}
  void reset() {
    par.assign(N, -1);
    rank.assign(N, 0);
    ncomp = N;
  }
  int size() const { return ncomp; }
  int find_rep(int u) { return par[u] < 0 ? u : par[u] = find_rep(par[u]); }
  bool union_rep(int u, int v) {
    int u_root = find_rep(u);
    int v_root = find_rep(v);
    if (u_root == v_root) return false;
    if (rank[u_root] > rank[v_root])
      par[v_root] = u_root;
    else {
      par[u_root] = v_root;
      if (rank[u_root] == rank[v_root]) rank[v_root] = rank[u_root] + 1;
    }
    --ncomp;
    return true;
  }
};
struct Edge {
  int from, to;
  bool operator<(const Edge &o) const {
    if (from != o.from) return from != o.from;
    return to != o.to;
  }
};
vector<vector<int>> og(1010), g(1010);
int main() {
  int n;
  cin >> n;
  vector<Edge> edges;
  DisjointSet ds(n);
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    u--, v--;
    edges.push_back({u, v});
  }
  int cnt = 0;
  vector<Edge> unused;
  set<Edge> leaders;
  int k = -1;
  for (int i = 0; i < edges.size(); i++) {
    if (!ds.union_rep(edges[i].from, edges[i].to)) {
      cnt++;
      unused.push_back(edges[i]);
    }
    k = ds.find_rep(edges[i].from);
  }
  for (int i = 1; i < n; i++)
    if (k != ds.find_rep(i)) leaders.insert({k, ds.find_rep(i)});
  cout << cnt << "\n";
  for (auto &t : leaders) {
    if (!unused.size()) break;
    cout << unused.back().from + 1 << " " << unused.back().to + 1 << " "
         << t.from + 1 << " " << t.to + 1 << "\n";
    unused.pop_back();
  }
  return 0;
}
