#include <bits/stdc++.h>
using namespace std;
vector<int> g[2000];
int vis[2000];
vector<pair<int, int> > cyc;
void dfs(int x, int scc) {
  if (vis[x] > 0) {
    return;
  }
  vis[x] = 1;
  int j;
  for (int i = 0; i < g[x].size(); i++) {
    for (j = 0; j < g[g[x][i]].size(); j++) {
      if (g[g[x][i]][j] == x) {
        g[g[x][i]].erase(g[g[x][i]].begin() + j);
      }
    }
    if (vis[g[x][i]] > 0) {
      cyc.push_back(make_pair(x, g[x][i]));
    }
    dfs(g[x][i], scc);
  }
  return;
}
int main() {
  int n, i, j, x, y, t, u;
  cin >> n;
  for (i = 0; i < n - 1; i++) {
    cin >> x >> y;
    x--;
    y--;
    g[x].push_back(y);
    g[y].push_back(x);
  }
  memset(vis, 0, sizeof(vis));
  int scc = 0, days = 0;
  cyc.clear();
  vector<int> store;
  store.clear();
  for (i = 0; i < n; i++) {
    if (vis[i] == 0) {
      scc++;
      store.push_back(i);
      dfs(i, scc);
    }
  }
  if (scc == 1) {
    cout << "0\n";
    return 0;
  }
  j = 1;
  vector<pair<pair<int, int>, pair<int, int> > > v;
  v.clear();
  for (i = 0; i < cyc.size(); i++) {
    v.push_back(make_pair(make_pair(cyc[i].first + 1, cyc[i].second + 1),
                          make_pair(store[j - 1] + 1, store[j] + 1)));
    j++;
  }
  cout << v.size() << "\n";
  for (i = 0; i < v.size(); i++) {
    cout << v[i].first.first << " " << v[i].first.second << " "
         << v[i].second.first << " " << v[i].second.second << "\n";
  }
  return 0;
}
