#include <bits/stdc++.h>
using namespace std;
const int inf = 1999999999;
const double pi = acos(-1.0);
const double eps = 1e-10;
char gc() {
  char c;
  while (isspace(c = getchar()))
    ;
  return c;
}
int gs(char* s) {
  gets(s);
  int l = strlen(s);
  while (l && isspace(s[l - 1])) s[--l] = 0;
  return l;
}
vector<int> adj[1000];
vector<pair<int, int> > ins, rem;
bool vis[1000], pres[1000];
int s, e;
void dfs(int a, int p) {
  pres[a] = true;
  for (int i = 0; i < ((int)(adj[a]).size()); i++) {
    int b = adj[a][i];
    if (!vis[b]) {
      vis[b] = true;
      dfs(b, a);
    } else if (p != b && pres[b]) {
      s = a;
      e = b;
    }
  }
  pres[a] = false;
}
void flood(int a) {
  for (int i = 0; i < ((int)(adj[a]).size()); i++) {
    int b = adj[a][i];
    if (!vis[b]) {
      vis[b] = true;
      flood(b);
    }
  }
}
int main() {
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    adj[a].push_back(b);
    adj[b].push_back(a);
  }
  while (true) {
    memset((vis), (false), sizeof(vis));
    memset((pres), (false), sizeof(pres));
    s = -1;
    for (int i = 0; i < n; i++) {
      if (!vis[i]) {
        vis[i] = true;
        dfs(i, -1);
        if (s != -1) break;
      }
    }
    if (s == -1) break;
    adj[s].erase(find((adj[s]).begin(), (adj[s]).end(), e));
    adj[e].erase(find((adj[e]).begin(), (adj[e]).end(), s));
    rem.push_back(make_pair(s, e));
  }
  {
    memset((vis), (false), sizeof(vis));
    vector<int> a;
    for (int i = 0; i < n; i++) {
      if (!vis[i]) {
        vis[i] = true;
        a.push_back(i);
        flood(i);
      }
    }
    for (int i = 0; i < ((int)(a).size()) - 1; i++)
      ins.push_back(make_pair(a[i], a[i + 1]));
  }
  cout << ((int)(ins).size()) << ('\n');
  for (int i = 0; i < ((int)(ins).size()); i++)
    cout << rem[i].first + 1 << ' ' << rem[i].second + 1 << ' '
         << ins[i].first + 1 << ' ' << ins[i].second + 1 << ' ';
  return 0;
}
