#include <bits/stdc++.h>
using namespace std;
int n, a, b, compLeader[1002], compAmount, vis[1002], myComp[1002];
vector<int> graph[1002];
void visit(int node, int leader) {
  myComp[node] = compAmount;
  for (int son : graph[node]) {
    if (!myComp[son]) visit(son, leader);
  }
}
void CountLeaders() {
  int t = -1;
  for (int i = 1; i <= n; i++) {
    if (!myComp[i]) {
      compLeader[++compAmount] = i, visit(i, i), t++;
    }
  }
  cout << t << "\n";
}
bool StillConnected(int node, int a, int b, int color) {
  vis[node] = color;
  if (node == b) return true;
  bool ans = false;
  for (int son : graph[node]) {
    if (son && (node != a || son != b) && vis[son] != color &&
        (node != b || son != a)) {
      ans |= StillConnected(son, a, b, color);
    }
  }
  return ans;
}
void Disconnect(int a, int b) {
  for (int i = 0; i < graph[a].size(); i++)
    if (graph[a][i] == b) graph[a][i] = 0;
}
int parent[1002], sz[1002];
void initialize() {
  for (int i = 1; i <= n; i++) parent[i] = i, sz[i] = 1;
}
int find(int x) {
  while (parent[x] != x) x = parent[x] = parent[parent[x]];
  return x;
}
bool joined(int a, int b) { return find(a) == find(b); }
void join(int a, int b) {
  int aRoot = find(a), bRoot = find(b);
  if (sz[aRoot] > sz[bRoot]) swap(aRoot, bRoot);
  sz[bRoot] += sz[aRoot];
  parent[aRoot] = bRoot;
}
void PrintNewRoads() {
  int color = 1;
  for (int i = 1; i <= n; i++) {
    for (int son : graph[i]) {
      int chosenComp = 1;
      while (joined(chosenComp, myComp[i])) chosenComp++;
      if (chosenComp <= compAmount && StillConnected(i, i, son, color++)) {
        join(chosenComp, myComp[i]);
        cout << i << " " << son << " " << i << " " << compLeader[chosenComp]
             << "\n";
        Disconnect(i, son), Disconnect(son, i);
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  initialize();
  for (int i = 1; i < n; i++)
    cin >> a >> b, graph[a].push_back(b), graph[b].push_back(a);
  CountLeaders();
  PrintNewRoads();
  return 0;
}
