#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e3 + 10;
int n, from[MAXN], to[MAXN];
vector<int> adj[MAXN];
int cv;
bool flag, vis[MAXN], del[MAXN];
vector<pair<pair<int, int>, pair<int, int>>> ans;
pair<pair<int, int>, pair<int, int>> temp;
void dfs(int v) {
  if (vis[v]) return;
  vis[v] = 1;
  cv++;
  for (int e : adj[v])
    if (!del[e]) dfs(from[e] ^ to[e] ^ v);
}
void go(int v, int p = -1, int ee = -1) {
  if (vis[v]) {
    temp.first.first = p + 1, temp.first.second = v + 1;
    del[ee] = 1;
    flag = 1;
    return;
  }
  vis[v] = 1;
  for (int e : adj[v]) {
    if (del[e]) continue;
    int u = from[e] ^ to[e] ^ v;
    if (u ^ p) go(u, v, e);
    if (flag) return;
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    cin >> from[i] >> to[i], from[i]--, to[i]--;
    adj[from[i]].push_back(i);
    adj[to[i]].push_back(i);
  }
  int sz = n - 1;
  bool changed = 1;
  while (changed) {
    changed = 0;
    memset(vis, 0, sizeof(vis));
    cv = 0;
    dfs(0);
    if (cv ^ n) {
      for (int i = 0; i < n; i++)
        if (!vis[i]) {
          temp.second.first = 1, temp.second.second = i + 1;
          from[sz] = 0, to[sz] = i;
          adj[0].push_back(sz);
          adj[i].push_back(sz);
          sz++;
          break;
        }
      changed = 1;
      memset(vis, 0, sizeof(vis));
      flag = 0;
      for (int i = 0; !flag && i < n; i++)
        if (!vis[i]) go(i);
      ans.push_back(temp);
    }
  }
  cout << ans.size() << "\n";
  for (auto v : ans)
    cout << v.first.first << " " << v.first.second << " " << v.second.first
         << " " << v.second.second << "\n";
  return 0;
}
