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;
string filename = "test";
vector<int> adj[1001];
int l[1001];
void mark(int s, int m) {
  l[s] = m;
  for (int _n(adj[s].size()), i(0); i < _n; i++)
    if (l[adj[s][i]] != m) {
      mark(adj[s][i], m);
    }
}
int us[1001];
vector<pair<int, int> > e;
void go(int s, int p) {
  us[s] = 1;
  for (int _n(adj[s].size()), i(0); i < _n; i++) {
    int j = adj[s][i];
    if (j == p) continue;
    if (us[j] == 1) {
      e.push_back(pair<int, int>(s + 1, j + 1));
    } else if (!us[j])
      go(j, s);
  }
  us[s] = 2;
}
int main() {
  int n;
  cin >> n;
  for (int _n(n - 1), i(0); i < _n; i++) {
    int x, y;
    cin >> x >> y;
    --x;
    --y;
    adj[x].push_back(y);
    adj[y].push_back(x);
  }
  for (int _n(n), i(0); i < _n; i++) l[i] = i;
  vector<pair<int, int> > res1;
  vector<pair<int, int> > res2;
  for (int _n(n), i(0); i < _n; i++)
    if (l[i] == i) {
      mark(i, i);
    }
  vector<int> vl;
  for (int _n(n), i(0); i < _n; i++) {
    vl.push_back(l[i]);
  }
  sort(((vl)).begin(), ((vl)).end()),
      (vl).erase(unique(((vl)).begin(), ((vl)).end()), (vl).end());
  if (vl.size() > 1) {
    memset(us, 0, sizeof(us));
    e.clear();
    for (int _n(n), i(0); i < _n; i++)
      if (!us[i]) go(i, -1);
    for (int _n(e.size()), i(0); i < _n; i++) {
      res1.push_back(e[i]);
      int l1 = vl[i];
      int l2 = vl[i + 1];
      int i1 = -1, i2 = -1;
      for (int _n(n), i(0); i < _n; i++)
        if (l[i] == l1) {
          i1 = i;
          break;
        }
      for (int _n(n), i(0); i < _n; i++)
        if (l[i] == l2) {
          i2 = i;
          break;
        }
      res2.push_back(pair<int, int>(i1 + 1, i2 + 1));
    }
  }
  cout << res1.size() << endl;
  for (int _n(res1.size()), i(0); i < _n; i++) {
    cout << res1[i].first << " " << res1[i].second << " " << res2[i].first
         << " " << res2[i].second << endl;
  }
}
