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 = 1 * 1000 + 10;
int n, u, v, in1, in2, khas[maxn], s = 1005, f = 1006, t[2 * maxn];
vector<int> vec[maxn], mlf[maxn];
bool mark[maxn], check[maxn][maxn];
pair<int, int> tmp;
vector<pair<int, int> > kharab, abad;
queue<pair<int, int> > q;
void bfs() {
  while (!q.empty()) {
    tmp = q.front();
    mlf[in1].push_back(tmp.first);
    q.pop();
    for (int i = 0; i < ((int(vec[tmp.first].size()))); i++) {
      if (!mark[vec[tmp.first][i]]) {
        mark[vec[tmp.first][i]] = 1;
        q.push(make_pair(vec[tmp.first][i], tmp.first));
      } else if (vec[tmp.first][i] != tmp.second &&
                 !check[tmp.first][vec[tmp.first][i]]) {
        check[tmp.first][vec[tmp.first][i]] =
            check[vec[tmp.first][i]][tmp.first] = 1;
        khas[tmp.first]++;
        u++;
        kharab.push_back(make_pair(tmp.first, vec[tmp.first][i]));
      }
    }
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    cin >> u >> v;
    vec[u].push_back(v);
    vec[v].push_back(u);
  }
  for (int i = 1; i <= n; i++) {
    if (!mark[i]) {
      mark[i] = 1;
      u = 0;
      q.push(make_pair(i, -1));
      bfs();
      if (u == 0) {
        t[f] = in1;
        f++;
      } else {
        t[s] = in1;
        s--;
      }
      in1++;
    }
  }
  in2 = s + 2;
  for (int i = s + 1; i < f; i++) {
    for (int j = 0; j < ((int(mlf[t[i]].size()))); j++) {
      for (int k = 0; k < khas[mlf[t[i]][j]]; k++) {
        abad.push_back(make_pair(mlf[t[i]][j], mlf[t[in2]][0]));
        in2++;
      }
    }
  }
  cout << ((int(kharab.size()))) << endl;
  for (int i = 0; i < ((int(kharab.size()))); i++)
    cout << kharab[i].first << " " << kharab[i].second << " " << abad[i].first
         << " " << abad[i].second << endl;
  return 0;
}
