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;
int n, e, u, cur, ans;
vector<int> v[1005], v2;
vector<pair<int, int> > v1[1005];
bool b[1005];
map<pair<int, int>, bool> mp;
void solve(int a, int l) {
  b[a] = true;
  for (int i = 0; i < v[a].size(); i++) {
    if (b[v[a][i]] == true) {
      if (v[a][i] != l) {
        if (mp[{a, v[a][i]}] == false) v1[cur].push_back({a, v[a][i]});
        mp[{a, v[a][i]}] = true;
        mp[{v[a][i], a}] = true;
      }
    } else {
      solve(v[a][i], a);
    }
  }
}
int main() {
  cin >> n;
  for (int i = 1; i <= n - 1; i++) {
    cin >> u >> e;
    v[e].push_back(u);
    v[u].push_back(e);
  }
  for (int i = 1; i <= n; i++) {
    if (b[i] == false) {
      cur = i;
      v2.push_back(i);
      ans++;
      solve(i, 0);
    }
  }
  for (int i = 1; i <= n; i++) {
    b[i] = false;
  }
  mp.clear();
  cout << ans - 1 << endl;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < v1[i].size(); j++) {
      for (int k = 0; k < v2.size(); k++) {
        if (b[k] == false && v2[k] != i && mp[{i, v2[k]}] == false) {
          cout << v1[i][j].first << " " << v1[i][j].second << " "
               << v1[i][j].first << " " << v2[k] << endl;
          b[k] = true;
          mp[{i, v2[k]}] == true;
          mp[{v2[k], i}] == true;
          break;
        }
      }
    }
  }
}
