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;
vector<int> adj[10000 + 1];
bool vis[10000 + 5];
vector<pair<int, int>> dups;
int dcou = 0;
struct obj {
  vector<pair<int, int>> dup;
  int dco = 0;
};
map<pair<int, int>, bool> check;
void dfs(int a, int ls) {
  vis[a] = 1;
  for (auto& a1 : adj[a]) {
    if (a1 != a && a1 != ls) {
      if (vis[a1]) {
        if (check[{a1, a}] || check[{a, a1}]) continue;
        dcou++;
        dups.push_back({a, a1});
        check[{a, a1}] = 1;
      } else {
        dfs(a1, a);
      }
    }
  }
}
int main() {
  memset(vis, 0, sizeof(vis));
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    adj[a].push_back(b);
    adj[b].push_back(a);
  }
  map<int, obj> info;
  vector<int> tre;
  vector<int> grp;
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      dcou = 0;
      dups.clear();
      check.clear();
      dfs(i, 0);
      obj o1;
      o1.dco = dcou;
      o1.dup = dups;
      info[i] = o1;
      if (dcou == 0) {
        tre.push_back(i);
      } else {
        grp.push_back(i);
      }
    }
  }
  int ans = 0;
  vector<pair<int, pair<int, pair<int, int>>>> an1;
  while (!grp.empty()) {
    int in1 = grp.back();
    int in2 = tre.back();
    grp.pop_back();
    tre.pop_back();
    ans++;
    pair<int, int> tmp = info[in1].dup.back();
    info[in1].dup.pop_back();
    int de = --info[in1].dco;
    an1.push_back({tmp.first, {tmp.second, {in1, in2}}});
    if (de == 0) {
      tre.push_back(in2);
    } else {
      grp.push_back(in1);
    }
  }
  cout << ans << "\n";
  for (auto& a1 : an1) {
    cout << a1.first << " " << a1.second.first << " "
         << " " << a1.second.second.first << " " << a1.second.second.second
         << "\n";
  }
  return 0;
}
