#include <bits/stdc++.h>
using namespace std;
int A;
int gcd(int a, int b) {
  if (a < b) swap(a, b);
  if (a % b == 0) return b;
  return gcd(b, a % b);
}
pair<int, int> find(vector<pair<int, int> > &sets, int a) {
  if (sets[a].first != a) sets[a].first = find(sets, sets[a].first).first;
  return sets[a];
}
void unions(vector<pair<int, int> > &sets, int a, int b) {
  pair<int, int> p = find(sets, a);
  pair<int, int> q = find(sets, b);
  if (p.second < q.second) swap(p, q);
  sets[q.first].first = p.first;
  if (p.second == q.second) sets[p.first].second++;
}
void solve() {
  long long int n, i, a, b, x, y, a1, b1;
  cin >> n;
  vector<pair<int, int> > sets(n, {0, 1});
  pair<int, int> p;
  for (i = 0; i < n; i++) sets[i].first = i;
  vector<pair<int, pair<int, int> > > v;
  for (i = 0; i < n - 1; i++) {
    cin >> a >> b;
    a--;
    b--;
    p = find(sets, a);
    a1 = p.first;
    x = p.second;
    p = find(sets, b);
    b1 = p.first;
    y = p.second;
    if (a1 != b1)
      unions(sets, a1, b1);
    else {
      v.push_back({a1, {a + 1, b + 1}});
    }
  }
  set<int> s;
  for (i = 0; i < n; i++) {
    s.insert(find(sets, sets[i].first).first);
  }
  auto itr = s.begin();
  x = *itr + 1;
  itr++;
  cout << v.size() << "\n";
  for (i = 0, itr; i < v.size(); i++, itr++)
    cout << v[i].second.first << " " << v[i].second.second << " " << x << " "
         << *itr + 1 << "\n";
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int T = 1;
  while (T--) {
    solve();
  }
  return 0;
}
