#include <bits/stdc++.h>
using namespace std;
const int N = 1005;
vector<int> graph[N], p;
vector<pair<int, int> > not_tree_edge;
int n;
bool was[N], fordel[N][N];
void dfs(int v) {
  was[v] = 1;
  for (int i = 0; i < (int)(graph[v]).size(); i++) {
    int to = graph[v][i];
    if (!was[to]) {
      fordel[v][to] = fordel[to][v] = false;
      dfs(to);
    }
  }
}
int main() {
  cin >> n;
  for (int i = 1; i < n; i++) {
    int u, v;
    cin >> u >> v;
    graph[u].push_back(v);
    graph[v].push_back(u);
    fordel[u][v] = fordel[v][u] = true;
  }
  for (int i = 1; i <= n; i++) {
    if (!was[i]) {
      p.push_back(i);
      dfs(i);
    }
  }
  for (int i = 1; i <= n; i++)
    for (int j = 0; j < (int)(graph[i]).size(); j++) {
      int to = graph[i][j];
      if (i < to)
        if (fordel[i][to]) not_tree_edge.push_back(make_pair(i, to));
    }
  cout << (int)(p).size() - 1 << endl;
  for (int i = 1; i < (int)(p).size(); i++)
    cout << not_tree_edge[i - 1].first << " " << not_tree_edge[i - 1].second
         << " " << p[0] << " " << p[i] << endl;
  return 0;
}
