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;
typedef struct _edge {
  int root;
  int source;
  int destination;
} edge;
void dfs(vector<int> *edges, vector<edge> &edge_to_cut, vector<edge> &blocked,
         vector<int> &visit, int cur, int pre, bool &is_cycle, int root) {
  if (visit[cur] == 0) {
    visit[cur] = 1;
    for (int next : edges[cur]) {
      if (visit[next] == 0) {
        dfs(edges, edge_to_cut, blocked, visit, next, cur, is_cycle, root);
      } else if (next != pre) {
        bool is_blocked = false;
        for (edge b : blocked) {
          if (b.source == cur && b.destination == next) {
            is_blocked = true;
          }
        }
        if (!is_blocked) {
          edge e = {root, cur, next};
          edge_to_cut.push_back(e);
          is_cycle = true;
          edge blocking = {root, next, cur};
          blocked.push_back(blocking);
        }
      }
    }
  }
}
int main(void) {
  int n;
  cin >> n;
  vector<int> *edges = new vector<int>[n];
  for (int i = 0; i < n - 1; i++) {
    int s, d;
    cin >> s >> d;
    edges[s - 1].push_back(d - 1);
    edges[d - 1].push_back(s - 1);
  }
  vector<int> visit(n, 0);
  vector<int> dest;
  vector<edge> edges_to_cut;
  vector<edge> blocked;
  for (int i = 0; i < n; i++) {
    if (visit[i] == 0) {
      bool is_cycle = false;
      dfs(edges, edges_to_cut, blocked, visit, i, -1, is_cycle, i);
      dest.push_back(i);
    }
  }
  if (edges_to_cut.size() == 0) {
    cout << 0 << endl;
    return 0;
  }
  int days = edges_to_cut.size();
  cout << days << endl;
  for (int i = 0; i < days; i++) {
    edge e = edges_to_cut[i];
    cout << e.source + 1 << ' ' << e.destination + 1 << ' ' << dest[i] + 1
         << ' ' << dest[i + 1] + 1 << endl;
  }
  return 0;
}
