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 sz = 1e3 + 9;
int n;
vector<int> v[sz];
int first, t;
vector<int> tree;
vector<pair<int, int> > e, tmp, ans;
int visited[sz];
void dfs(int i, int p) {
  visited[i] = 1;
  for (auto u : v[i]) {
    if (u == p) continue;
    if (visited[u] == 2) continue;
    if (!visited[u])
      dfs(u, i);
    else
      tmp.push_back(make_pair(u, i));
  }
  visited[i] = 2;
}
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n - 1; i++) {
    scanf("%d %d", &first, &t);
    v[first].push_back(t);
    v[t].push_back(first);
  }
  for (int i = 1; i <= n; i++) {
    if (visited[i]) continue;
    tmp.clear();
    dfs(i, i);
    if (tmp.empty())
      tree.push_back(i);
    else
      for (auto u : tmp) e.push_back(u);
  }
  for (auto u : e) {
    auto to = tree.back();
    tree.pop_back();
    ans.push_back(make_pair(u.first, to));
  }
  reverse(ans.begin(), ans.end());
  printf("%d\n", ans.size());
  for (auto u : e) {
    printf("%d %d ", u.first, u.second);
    auto k = ans.back();
    ans.pop_back();
    printf("%d %d\n", k.first, k.second);
  }
  return 0;
}
