#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};
int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dy8[] = {1, -1, -1, 0, 1, -1, 0, 1};
int kx8[] = {1, 1, 2, 2, -1, -1, -2, -2};
int ky8[] = {2, -2, 1, -1, 2, -2, 1, -1};
long long bigmod(long long a, long long b, long long c) {
  if (b == 0) return 1 % c;
  long long x = bigmod(a, b / 2, c);
  x = (x * x) % c;
  if (b % 2 == 1) x = (x * a) % c;
  return x;
}
long long mod_inverse(long long a, long long mod) {
  return bigmod(a, mod - 2, mod);
}
int par[1005];
vector<pair<int, int> > vc;
long long unfn(int a) {
  if (a == par[a]) return a;
  return par[a] = unfn(par[a]);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) par[i] = i;
  for (int i = 0; i < (n - 1); i++) {
    int x, y;
    cin >> x >> y;
    int u = unfn(x);
    int v = unfn(y);
    if (u == v) {
      vc.push_back({x, y});
      continue;
    }
    par[u] = v;
  }
  int pre = 1;
  cout << vc.size() << endl;
  int id = 0;
  for (int i = 1; i <= n; i++) {
    int u = unfn(i);
    int v = unfn(pre);
    if (u != v) {
      cout << vc[id].first << " " << vc[id].second << " " << u << " " << v
           << endl;
      id++;
      par[v] = u;
    }
    pre = u;
  }
  return 0;
}
