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>
int dx[] = {0, -1, 0, 1, -1, 1, -1, 1};
int dy[] = {-1, 0, 1, 0, 1, -1, -1, 1};
const int N = 3e5 + 5, oo = 1e9, mod = 1e9 + 7;
const double pi = acos(-1);
using namespace std;
vector<int> g[N];
set<pair<int, int>> edges;
map<int, int> comp;
map<int, vector<int>> mp;
int scc;
bool vis[N];
void dfs(int node, int par) {
  vis[node] = true;
  comp[node] = scc;
  mp[scc].push_back(node);
  for (auto child : g[node]) {
    if (!vis[child])
      dfs(child, node);
    else if (child != par) {
      if (node > child)
        edges.insert({child, node});
      else
        edges.insert({node, child});
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      scc++;
      dfs(i, 0);
    }
  }
  cout << (int)(edges).size() << '\n';
  set<int> st, used;
  for (int i = 1; i <= scc; i++) st.insert(i);
  for (auto it : edges) {
    cout << it.first << ' ' << it.second << ' ';
    if (st.find(comp[it.first]) != st.end()) st.erase(comp[it.first]);
    if (used.find(comp[it.first]) != used.end() || used.empty()) {
      auto i = st.begin();
      cout << it.first << ' ' << mp[*i].back() << '\n';
      st.erase(i);
      used.insert(*i);
      used.insert(comp[it.first]);
    } else {
      auto i = used.begin();
      cout << it.first << ' ' << mp[*i].back() << '\n';
      used.insert(comp[it.first]);
      st.erase(comp[it.first]);
    }
  }
  return 0;
}
