#include <bits/stdc++.h>
using namespace std;
typedef struct _edge {
  int root;
  int source;
  int destination;
} edge;
void dfs(vector<int> *edges, vector<edge> &edge_to_cut, vector<edge> &blocked,
         vector<int> &visit, int cur, int pre, bool &is_cycle, int root) {
  if (visit[cur] == 0) {
    visit[cur] = 1;
    for (int next : edges[cur]) {
      if (visit[next] == 0) {
        dfs(edges, edge_to_cut, blocked, visit, next, cur, is_cycle, root);
      } else if (next != pre) {
        bool is_blocked = false;
        for (edge b : blocked) {
          if (b.source == cur && b.destination == next) {
            is_blocked = true;
          }
        }
        if (!is_blocked) {
          edge e = {root, cur, next};
          edge_to_cut.push_back(e);
          is_cycle = true;
          edge blocking = {root, next, cur};
          blocked.push_back(blocking);
        }
      }
    }
  }
}
int main(void) {
  int n;
  cin >> n;
  vector<int> *edges = new vector<int>[n];
  for (int i = 0; i < n - 1; i++) {
    int s, d;
    cin >> s >> d;
    edges[s - 1].push_back(d - 1);
    edges[d - 1].push_back(s - 1);
  }
  vector<int> visit(n, 0);
  vector<int> dest;
  vector<edge> edges_to_cut;
  vector<edge> blocked;
  for (int i = 0; i < n; i++) {
    if (visit[i] == 0) {
      bool is_cycle = false;
      dfs(edges, edges_to_cut, blocked, visit, i, -1, is_cycle, i);
      dest.push_back(i);
    }
  }
  if (edges_to_cut.size() == 0) {
    cout << 0 << endl;
    return 0;
  }
  int days = edges_to_cut.size();
  cout << days << endl;
  for (int i = 0; i < days; i++) {
    edge e = edges_to_cut[i];
    cout << e.source + 1 << ' ' << e.destination + 1 << ' ' << dest[i] + 1
         << ' ' << dest[i + 1] + 1 << endl;
  }
  return 0;
}
