#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3")
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast,unroll-loops,no-stack-protector,fast-math")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
const char nl = '\n';
const long long mod = 1e9 + 7;
const long long N = 1e5 + 5;
vector<long long> parent(N), size1(N);
long long find_set(long long v) {
  if (v == parent[v]) return v;
  return parent[v] = find_set(parent[v]);
}
void make_set(long long v) {
  parent[v] = v;
  size1[v] = 1;
}
void union_sets(long long a, long long b) {
  if (a != b) {
    if (size1[a] < size1[b]) swap(a, b);
    parent[b] = a;
    size1[a] += size1[b];
  }
}
void accio_ac(void) {
  long long n;
  cin >> n;
  for (long long i = 1; i <= n; i++) {
    make_set(i);
  }
  vector<pair<long long, long long> > extra, not_there;
  for (long long i = 1; i < n; i++) {
    long long u, v, x, y;
    cin >> u >> v;
    x = u;
    y = v;
    u = find_set(u);
    v = find_set(v);
    if (u == v) {
      extra.emplace_back(make_pair(x, y));
    } else {
      union_sets(u, v);
    }
  }
  for (long long i = 1; i <= n; i++) {
    for (long long j = i + 1; j <= n; j++) {
      long long a = find_set(i);
      long long b = find_set(j);
      if (a != b) {
        not_there.emplace_back(make_pair(i, j));
        union_sets(a, b);
      }
    }
  }
  long long k = not_there.size();
  cout << k << '\n';
  for (long long i = 0; i < k; i++) {
    cout << extra[i].first << ' ' << extra[i].second << ' '
         << not_there[i].first << ' ' << not_there[i].second << nl;
  }
}
int32_t main(void) {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(10);
  long long __ = 1;
  while (__--) {
    accio_ac();
  }
  return 0;
}
