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>
const int M = 1e9 + 7, N = 2003;
using namespace std;
vector<pair<long long, long long> > p0;
set<pair<long long, long long> > p, p1;
vector<long long> v[N];
bool u[N];
int s[N];
void dfs(int i, int j, int q) {
  u[i] = 1;
  s[i] = q;
  for (int l : v[i])
    if (l != j) {
      if (u[l]) {
        int l0 = max(l, i), i0 = min(i, l);
        p.insert({l0, i0});
      } else
        dfs(l, i, q);
    }
}
int main() {
  int n;
  cin >> n;
  for (int i = 1, x, y; i < n; i++) {
    cin >> x >> y;
    v[x].push_back(y);
    v[y].push_back(x);
  }
  int j = 0, k = 0;
  for (int i = 1; i <= n; i++) {
    if (!u[i]) j++, dfs(i, 0, j);
  }
  for (int i = 1; i <= n; i++) p0.push_back({s[i], i});
  sort(p0.begin(), p0.end());
  cout << j - 1 << "\n";
  for (int i = 1; i < n; i++) {
    if (p0[i].first != p0[i - 1].first) {
      k++;
      auto w = p.begin();
      p.erase(p.begin());
      cout << w->first << " " << w->second << " " << p0[i].second << " "
           << p0[i - 1].second << "\n";
    }
  }
  return 0;
}
