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 to_string(string s) { return '"' + s + '"'; }
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) res += ", ";
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
const int N = 1e4 + 3;
int n;
int parent[N];
int Size[N];
vector<pair<int, int>> cycle;
set<int> st;
int root(int x) {
  if (parent[x] == x) {
    return x;
  }
  return parent[x] = root(parent[x]);
}
void unify(int x, int y) {
  x = root(x);
  y = root(y);
  if (x == y) {
    return;
  }
  if (Size[x] < Size[y]) {
    swap(x, y);
  }
  parent[y] = x;
  Size[x] += Size[y];
}
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n;
  for (int i = 1; i <= n; i++) {
    parent[i] = i;
    Size[i] = 1;
  }
  for (int i = 1; i < n; i++) {
    int u, v;
    cin >> u >> v;
    if (root(u) == root(v)) {
      cycle.push_back({u, v});
    } else {
      unify(u, v);
    }
  }
  for (int i = 1; i <= n; i++) {
    st.insert(root(i));
  }
  int sz = cycle.size();
  cout << sz << '\n';
  for (int i = 0; i < sz; i++) {
    cout << cycle[i].first << ' ' << cycle[i].second << ' ';
    auto it = st.begin();
    cout << *it << ' ';
    ++it;
    cout << *it << '\n';
    st.erase(it);
  }
  42;
  42;
  return 0;
}
