#include <bits/stdc++.h>
using namespace std;
int parent[1005], size[1005];
vector<pair<int, int> > circle;
vector<int> lead;
void make_set(int n) {
  for (int i = 1; i <= n; ++i) {
    parent[i] = i;
    size[i] = 1;
  }
}
int find_root(int n) {
  if (parent[n] == n) return n;
  return parent[n] = find_root(parent[n]);
}
void merge(int a, int b) {
  if (size[a] < size[b]) swap(a, b);
  parent[b] = a;
  size[a] += size[b];
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  make_set(n);
  for (int i = 1; i < n; ++i) {
    int a, b;
    cin >> a >> b;
    int root_a = find_root(a);
    int root_b = find_root(b);
    if (root_a == root_b)
      circle.push_back(make_pair(a, b));
    else
      merge(root_a, root_b);
  }
  int res = 0;
  for (int i = 1; i <= n; ++i) {
    if (parent[i] == i) {
      res++;
      lead.push_back(i);
    }
  }
  cout << res - 1 << '\n';
  for (int i = 1; i < res; ++i) {
    cout << circle.back().first << ' ' << circle.back().second << ' '
         << lead.back() << ' ';
    circle.pop_back();
    lead.pop_back();
    cout << lead.back() << '\n';
  }
  return 0;
}
