#include <bits/stdc++.h>
using namespace std;
void init_code() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
set<pair<int, int>> es;
map<int, list<int>> m;
void add(int a, int b) {
  m[a].push_back(b);
  m[b].push_back(a);
}
int v[1001];
void dfs(int node) {
  v[node] = 1;
  for (int i : m[node]) {
    if (v[i] == 0) {
      es.erase({node, i});
      es.erase({i, node});
      dfs(i);
    }
  }
}
int main() {
  init_code();
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    add(a, b);
    es.insert({a, b});
  }
  vector<int> ans;
  for (int i = 1; i <= n; i++) {
    if (v[i] == 0) {
      ans.push_back(i);
      dfs(i);
    }
  }
  cout << es.size() << endl;
  vector<pair<int, int>> ans2, ans3;
  for (auto i : es) {
    ans2.push_back(i);
  }
  for (int i = 1; i < ans.size(); i++) {
    ans3.push_back({ans[i], ans[i - 1]});
  }
  for (int i = 0; i < ans2.size(); i++) {
    cout << ans2[i].first << " " << ans2[i].second << " " << ans3[i].first
         << " " << ans3[i].second << endl;
  }
}
