#include <bits/stdc++.h>
using namespace std;
int n;
const int mxN = 1000;
int prnt[mxN];
int rnk[mxN];
vector<int> adj[mxN];
vector<pair<int, int>> ans;
int vis[mxN];
int find(int i) {
  if (prnt[i] != i) prnt[i] = find(prnt[i]);
  return prnt[i];
}
void join(int x, int y) {
  int xroot = find(x);
  int yroot = find(y);
  if (rnk[xroot] < rnk[yroot])
    prnt[xroot] = yroot;
  else if (rnk[xroot] > rnk[yroot])
    prnt[yroot] = xroot;
  else {
    prnt[yroot] = xroot;
    rnk[xroot]++;
  }
}
void dfs(int u, int e) {
  for (auto ad = adj[u].begin(); ad != adj[u].end(); ad++) {
    if (*ad == e || vis[*ad]) continue;
    int adprnt = find(*ad);
    int uprnt = find(u);
    if (adprnt == uprnt) {
      ans.push_back(make_pair(u, *ad));
      continue;
    }
    join(*ad, u);
    dfs(*ad, u);
  }
  vis[u] = 1;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    adj[x].push_back(y);
    adj[y].push_back(x);
  }
  for (int i = 0; i < n; i++) {
    prnt[i] = i;
  }
  for (int i = 0; i < n; i++) {
    if (!vis[i]) {
      dfs(i, -1);
    }
  }
  cout << ans.size() << endl;
  int id = 0;
  for (int i = 0; i < n; i++) {
    for (int j = i + 1; j < n; j++) {
      int x = find(i);
      int y = find(j);
      if (x != y) {
        cout << ans[id].first + 1 << " " << ans[id].second + 1 << " " << i + 1
             << " " << j + 1 << endl;
        id++;
        join(i, j);
      }
    }
  }
  return 0;
}
