#include <bits/stdc++.h>
using namespace std;
vector<int> adj[10000 + 1];
bool vis[10000 + 5];
vector<pair<int, int>> dups;
int dcou = 0;
struct obj {
  vector<pair<int, int>> dup;
  int dco = 0;
};
map<pair<int, int>, bool> check;
void dfs(int a, int ls) {
  vis[a] = 1;
  for (auto& a1 : adj[a]) {
    if (a1 != a && a1 != ls) {
      if (vis[a1]) {
        if (check[{a1, a}] || check[{a, a1}]) continue;
        dcou++;
        dups.push_back({a, a1});
        check[{a, a1}] = 1;
      } else {
        dfs(a1, a);
      }
    }
  }
}
int main() {
  memset(vis, 0, sizeof(vis));
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    adj[a].push_back(b);
    adj[b].push_back(a);
  }
  map<int, obj> info;
  vector<int> tre;
  vector<int> grp;
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      dcou = 0;
      dups.clear();
      check.clear();
      dfs(i, 0);
      obj o1;
      o1.dco = dcou;
      o1.dup = dups;
      info[i] = o1;
      if (dcou == 0) {
        tre.push_back(i);
      } else {
        grp.push_back(i);
      }
    }
  }
  int ans = 0;
  vector<pair<int, pair<int, pair<int, int>>>> an1;
  while (!grp.empty()) {
    int in1 = grp.back();
    int in2 = tre.back();
    grp.pop_back();
    tre.pop_back();
    ans++;
    pair<int, int> tmp = info[in1].dup.back();
    info[in1].dup.pop_back();
    int de = --info[in1].dco;
    an1.push_back({tmp.first, {tmp.second, {in1, in2}}});
    if (de == 0) {
      tre.push_back(in2);
    } else {
      grp.push_back(in1);
    }
  }
  cout << ans << "\n";
  for (auto& a1 : an1) {
    cout << a1.first << " " << a1.second.first << " "
         << " " << a1.second.second.first << " " << a1.second.second.second
         << "\n";
  }
  return 0;
}
