#include <bits/stdc++.h>
using namespace std;
int F1[1009][1009];
int main() {
  int n;
  vector<int> V[1009];
  vector<pair<int, int> > B;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    V[a].push_back(b);
    V[b].push_back(a);
    B.push_back(pair<int, int>(a, b));
  }
  vector<int> V1;
  vector<pair<int, int> > V2;
  vector<int> T(n + 1, -1);
  vector<int> F(n + 1, 0);
  int kk = 1;
  for (int i = 1; i <= n; i++) {
    if (!F[i]) {
      F[i] = 1;
      V1.push_back(i);
      queue<int> S;
      S.push(i);
      while (!S.empty()) {
        int k = S.front();
        S.pop();
        for (typeof(V[k].begin()) it = V[k].begin(); it != V[k].end(); it++) {
          if (!F[*it]) {
            S.push(*it);
            F[*it] = 1;
            F1[k][*it] = 1;
          }
        }
      }
    }
  }
  for (typeof(B.begin()) it = B.begin(); it != B.end(); it++) {
    int k1 = it->first, k2 = it->second;
    if (!F1[k1][k2] && !F1[k2][k1]) V2.push_back(pair<int, int>(k1, k2));
  }
  int n1 = V1.size();
  cout << n1 - 1 << endl;
  for (int i = 0; i < n1 - 1; i++)
    cout << V2[i].first << " " << V2[i].second << " " << V1[i] << " "
         << V1[i + 1] << endl;
  return 0;
}
