#include <bits/stdc++.h>
struct union_find {
  struct node {
    int root, rank, size;
    node(int id = 0) : root(id), rank(0), size(1) {}
  };
  mutable std::vector<node> data;
  union_find(int SZ = 0) : data(SZ) { iota(data.begin(), data.end(), 0); }
  int find(int i) const {
    if (i != data[i].root) data[i].root = find(data[i].root);
    return data[i].root;
  }
  bool is_root(int i) const { return i == find(i); }
  node& root_node(int i) const { return data[find(i)]; }
  bool unite(int a, int b) {
    a = find(a), b = find(b);
    if (a == b) return false;
    if (data[a].rank < data[b].rank) std::swap(a, b);
    data[b].root = a;
    data[a].size += data[b].size;
    if (data[a].rank == data[b].rank) data[a].rank++;
    return true;
  }
  friend void pr(const union_find& u) {
    std::cout << "{";
    bool first = 1;
    for (int i = 0; i < int(u.data.size()); i++) {
      if (u.is_root(i)) {
        if (!first)
          std::cout << ", ";
        else
          first = 0;
        std::cout << "[ " << i << " | rank=" << u.data[i].rank
                  << " size=" << u.data[i].size << " ]";
      }
    }
    std::cout << "}";
  }
};
using namespace std;
int main() {
  int N;
  scanf("%d", &N);
  union_find uf(N);
  std::vector<pair<int, int>> redundant;
  for (int i = 0; i < N - 1; i++) {
    int u, v;
    scanf("%d %d", &u, &v);
    --u, --v;
    if (!uf.unite(u, v)) redundant.emplace_back(u, v);
  }
  std::vector<int> component_roots;
  for (int i = 0; i < N; i++)
    if (uf.is_root(i)) component_roots.push_back(i);
  assert(redundant.size() + 1 == component_roots.size());
  printf("%d\n", redundant.size());
  for (int i = 0; i < redundant.size(); i++) {
    printf("%d %d %d %d\n", redundant[i].first + 1, redundant[i].second + 1,
           component_roots[i] + 1, component_roots[i + 1] + 1);
  }
  return 0;
}
