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 MAXN = 2e3 + 10;
int n, from[MAXN], to[MAXN];
vector<int> adj[MAXN];
int cv;
bool flag, vis[MAXN], del[MAXN];
vector<pair<pair<int, int>, pair<int, int>>> ans;
pair<pair<int, int>, pair<int, int>> temp;
void dfs(int v) {
  if (vis[v]) return;
  vis[v] = 1;
  cv++;
  for (int e : adj[v])
    if (!del[e]) dfs(from[e] ^ to[e] ^ v);
}
void go(int v, int p = -1, int ee = -1) {
  if (vis[v]) {
    temp.first.first = p + 1, temp.first.second = v + 1;
    del[ee] = 1;
    flag = 1;
    return;
  }
  vis[v] = 1;
  for (int e : adj[v]) {
    if (del[e]) continue;
    int u = from[e] ^ to[e] ^ v;
    if (u ^ p) go(u, v, e);
    if (flag) return;
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    cin >> from[i] >> to[i], from[i]--, to[i]--;
    adj[from[i]].push_back(i);
    adj[to[i]].push_back(i);
  }
  int sz = n - 1;
  bool changed = 1;
  while (changed) {
    changed = 0;
    memset(vis, 0, sizeof(vis));
    cv = 0;
    dfs(0);
    if (cv ^ n) {
      for (int i = 0; i < n; i++)
        if (!vis[i]) {
          temp.second.first = 1, temp.second.second = i + 1;
          from[sz] = 0, to[sz] = i;
          adj[0].push_back(sz);
          adj[i].push_back(sz);
          sz++;
          break;
        }
      changed = 1;
      memset(vis, 0, sizeof(vis));
      flag = 0;
      for (int i = 0; !flag && i < n; i++)
        if (!vis[i]) go(i);
      ans.push_back(temp);
    }
  }
  cout << ans.size() << "\n";
  for (auto v : ans)
    cout << v.first.first << " " << v.first.second << " " << v.second.first
         << " " << v.second.second << "\n";
  return 0;
}
