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 N = 1000;
vector<int> G[N];
bool used[N];
pair<int, int> dfs(int u, int p = -1) {
  used[u] = true;
  for (int to : G[u]) {
    if (!used[to]) {
      auto res = dfs(to, u);
      if (res != make_pair(-1, -1)) return res;
    } else if (to != p) {
      return {u, to};
    }
  }
  return {-1, -1};
}
void dfs2(int u) {
  used[u] = true;
  for (int to : G[u]) {
    if (!used[to]) dfs2(to);
  }
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; ++i) {
    int u, v;
    cin >> u >> v;
    G[--u].push_back(--v);
    G[v].push_back(u);
  }
  vector<vector<int>> ans;
  while (true) {
    fill(used, used + n, 0);
    dfs2(0);
    if (all_of(used, used + n, [](bool i) { return i; })) break;
    fill(used, used + n, 0);
    int i = -1, j = -1;
    for (int u = 0; u < n; ++u) {
      if (!used[u]) {
        auto [x, y] = dfs(u);
        if (x != -1 && y != -1) {
          i = x;
          j = y;
          break;
        }
      }
    }
    if (i == -1 && j == -1) break;
    fill(used, used + n, 0);
    dfs2(0);
    int v = -1;
    for (int u = 0; u < n; ++u) {
      if (!used[u]) {
        v = u;
        break;
      }
    }
    assert(v != -1);
    G[0].push_back(v);
    G[v].push_back(0);
    ans.push_back({i, j, 0, v});
  }
  cout << ans.size() << "\n";
  for (int i = 0; i < int(ans.size()); ++i) {
    for (int j = 0; j < 4; ++j) cout << ans[i][j] + 1 << " ";
    cout << "\n";
  }
  return 0;
}
