#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;
}
