#include <bits/stdc++.h>
using namespace std;
int dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};
const int N = 2e3 + 5, M = 27;
int par[N], sz[N], n, m;
set<int> roots;
vector<pair<int, int>> edges;
int find(int x) {
  if (par[x] == x) return x;
  return par[x] = find(par[x]);
}
bool connect(int a, int b) {
  int u = find(a), v = find(b);
  if (u == v) {
    edges.push_back({a, b});
    return 0;
  }
  if (sz[u] > sz[v]) {
    par[v] = u;
    roots.erase(v);
    sz[u] += sz[v];
  } else {
    par[u] = v;
    roots.erase(u);
    sz[v] += sz[u];
  }
  return 1;
}
void init() {
  roots.clear();
  for (int(i) = 1; i <= (n); i++) {
    roots.insert(i);
    par[i] = i;
    sz[i] = 1;
  }
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  cin >> n;
  init();
  int a, b;
  for (int(i) = 0; i < (n - 1); i++) {
    cin >> a >> b;
    connect(a, b);
  }
  cout << edges.size() << "\n";
  vector<int> vv((roots).begin(), (roots).end());
  for (int(i) = 0; i < (vv.size() - 1); i++) {
    cout << edges[i].first << " " << edges[i].second << " " << vv[i] << " "
         << vv[i + 1] << "\n";
  }
}
