#include <bits/stdc++.h>
using namespace std;
const int MAX = 1010;
const int MAXN = 1e4 + 10;
const int MOD = 1e9 + 7;
const int inf = 1e9;
const double pi = acos(-1.0);
const double eps = 1e-6;
int f[MAX];
vector<pair<int, int> > a;
int findSet(int u) { return f[u] = (u == f[u] ? f[u] : findSet(f[u])); }
int Union(int u, int v) {
  u = findSet(u);
  v = findSet(v);
  if (u != v) {
    f[v] = u;
    return 1;
  }
  return 0;
}
int main() {
  ios::sync_with_stdio(false);
  int n, u, v;
  cin >> n;
  for (int i = 1; i <= n; i++) f[i] = i;
  for (int i = 1; i < n; i++) {
    cin >> u >> v;
    if (!Union(u, v)) a.push_back(make_pair(u, v));
  }
  cout << a.size() << endl;
  u = 1;
  while (u <= n && f[u] != u) u++;
  v = u + 1;
  for (int i = 0; i < a.size(); i++) {
    while (v <= n && f[v] != v) v++;
    cout << a[i].first << " " << a[i].second << " " << u << " " << v++ << endl;
  }
  return 0;
}
