#include <bits/stdc++.h>
using namespace std;
set<pair<int, int>> cont;
vector<bool> vis;
void dfs(int node, int p, set<int> adj[]) {
  if (vis[node]) {
    if (p != -1) {
      int tmp1 = p;
      int tmp2 = node;
      if (tmp1 > tmp2) swap(tmp1, tmp2);
      cont.insert({tmp1, tmp2});
    }
    return;
  }
  vis[node] = true;
  for (auto to : adj[node])
    if (to != p) dfs(to, node, adj);
}
int32_t main() {
  int n;
  cin >> n;
  set<int> adj[n + 1];
  vis.assign(n + 1, false);
  vector<int> points;
  for (int i = 1; i < n; i++) {
    int u, v;
    cin >> u >> v;
    adj[u].insert(v);
    adj[v].insert(u);
  }
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      points.push_back(i);
      dfs(i, -1, adj);
    }
  }
  if (points.size() <= 1) {
    cout << 0 << "\n";
    return 0;
  }
  cout << points.size() - 1 << "\n";
  for (int i = 1; i < points.size(); i++) {
    auto pr = *cont.begin();
    cont.erase(pr);
    cout << pr.first << " " << pr.second << " " << points[i - 1] << " "
         << points[i] << "\n";
  }
}
