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;
string alpha = "abcdefghijklmnopqrstuvwxyz";
const long long mod = 1e9 + 7;
const long long inf = 2e18 + 5;
const long long nax = 1010;
vector<unordered_set<long long>> g;
stack<long long> C;
stack<long long> UC;
vector<bool> visited(nax);
void dfs(long long u) {
  visited[u] = true;
  for (auto v : g[u]) {
    if (!visited[v]) {
      dfs(v);
    }
  }
}
bool has_cycle(long long u) {
  unordered_set<long long> visited;
  visited.insert(u);
  unordered_map<long long, long long> parent;
  parent[u] = u;
  vector<long long> frt = {u};
  vector<long long> nxt;
  while (!frt.empty()) {
    nxt.clear();
    for (auto u : frt) {
      for (auto v : g[u]) {
        if (!visited.count(v)) {
          parent[v] = u;
          visited.insert(v);
          nxt.push_back(v);
        } else if (parent[u] == v) {
          continue;
        } else {
          return true;
        }
      }
    }
    frt = nxt;
  }
  return false;
}
pair<long long, long long> get_edge(long long u) {
  unordered_set<long long> visited;
  visited.insert(u);
  unordered_map<long long, long long> parent;
  parent[u] = u;
  vector<long long> frt = {u};
  vector<long long> nxt;
  while (!frt.empty()) {
    nxt.clear();
    for (auto u : frt) {
      for (auto v : g[u]) {
        if (!visited.count(v)) {
          parent[v] = u;
          visited.insert(v);
          nxt.push_back(v);
        } else if (parent[u] == v) {
          continue;
        } else {
          return {v, u};
        }
      }
    }
    frt = nxt;
  }
  assert(false);
  return {0, 0};
}
long long cnt;
string res = "";
void unite(long long u, long long v) {
  cnt++;
  auto p = get_edge(u);
  long long a = p.first;
  long long b = p.second;
  g[b].erase(a);
  g[a].erase(b);
  g[b].insert(v);
  g[v].insert(b);
  res += to_string(a) + " " + to_string(b) + " " + to_string(b) + " " +
         to_string(v) + '\n';
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  g = vector<unordered_set<long long>>(n + 1);
  for (long long i = (0); i < (n - 1); i++) {
    long long u, v;
    cin >> u >> v;
    g[u].insert(v);
    g[v].insert(u);
  }
  vector<long long> roots;
  for (long long i = (1); i < (n + 1); i++) {
    if (!visited[i]) {
      roots.push_back(i);
      dfs(i);
    }
  }
  for (auto r : roots) {
    if (has_cycle(r)) {
      C.push(r);
    } else {
      UC.push(r);
    }
  }
  while (!C.empty()) {
    long long u, v;
    u = C.top();
    C.pop();
    v = UC.top();
    UC.pop();
    unite(u, v);
    if (has_cycle(u)) {
      C.push(u);
    } else {
      UC.push(u);
    }
  }
  cout << cnt << '\n';
  cout << res;
  return 0;
}
