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;
void err(istream_iterator<string> it) {}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << "=" << a << ", ";
  err(++it, args...);
}
long long int par[1007], x[1007][1007], sz[1007], n;
string s, S;
vector<pair<long long int, long long int> > v, v2;
long long int find_set(long long int in) {
  if (par[in] == in) return in;
  return par[in] = find_set(par[in]);
}
void union_set(long long int a, long long int b) {
  long long int pa = find_set(a), push_back = find_set(b);
  if (pa == push_back) return;
  if (sz[pa] < sz[push_back]) swap(pa, push_back);
  par[push_back] = pa;
  sz[pa] += sz[push_back];
}
int main() {
  {
    long long int a = 0, b = 0, c = 0, d, e, f, g, i, j, k, l, m, p, q, r, u, w,
                  t, tc = 1, in, loc, val, sz, lo, hi, mid, mn = 2134567891,
                  mx = 0, sum = 0, ans = 0;
    for (t = 1; t <= tc; t++) {
      cin >> n;
      for (i = 1; i <= n; i++) par[i] = i;
      for (i = 1; i <= n - 1; i++) {
        cin >> a >> b;
        union_set(a, b);
        x[a][b] = x[b][a] = 1;
      }
      for (i = 1; i <= n; i++) {
        for (j = i + 1; j <= n; j++) {
          if (i == j) continue;
          if (x[i][j] && find_set(i) == find_set(j)) {
            v.push_back({i, j});
          } else if (find_set(i) != find_set(j)) {
            v2.push_back({i, j});
            union_set(i, j);
          }
        }
      }
      cout << v2.size() << endl;
      for (i = 0; i < v2.size(); i++)
        cout << v[i].first << " " << v[i].second << " " << v2[i].first << " "
             << v2[i].second << endl;
    }
  }
  return 0;
}
