#include <bits/stdc++.h>
using namespace std;
const int ma = 100000 * 5 + 1000;
int n, m, a, b, pr[ma], szz[ma];
vector<int> edge[ma], nu1, nu2;
vector<pair<int, int> > edge1[ma];
string s;
vector<long long> qq, q;
long long mp(long long a, long long b) { return a * 1000000 + b; }
void dfs(int nu, int pr) {
  for (auto v : edge[nu])
    if (v != pr) dfs(v, nu);
  if (pr != -1) nu1.push_back(nu), nu2.push_back(pr);
}
int get_root(int a) {
  if (pr[a] == a) return a;
  pr[a] = get_root(pr[a]);
  return pr[a];
}
void unite(int a, int b) {
  a = get_root(a);
  b = get_root(b);
  if (szz[a] > szz[b]) swap(a, b);
  pr[a] = b;
  szz[b] += szz[a];
  for (int i = 0; i < int((int)edge1[a].size()); i++) {
    edge1[b].push_back(edge1[a][i]);
  }
  edge1[a].clear();
}
int main() {
  iostream::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  for (int i = 0; i < int(n - 1); i++) {
    cin >> a >> b;
    a--;
    b--;
    q.push_back(mp(min(a, b), max(a, b)));
    edge[a].push_back(b);
    edge[b].push_back(a);
  }
  sort(q.begin(), q.end());
  int ans = n - 1;
  for (int i = 0; i < int(n - 1); i++) {
    cin >> a >> b;
    a--;
    b--;
    if (binary_search(q.begin(), q.end(), mp(min(a, b), max(a, b)))) ans--;
    qq.push_back(mp(min(a, b), max(a, b)));
    edge1[a].push_back({b, a});
    edge1[b].push_back({a, b});
  }
  for (int i = 0; i < int(n); i++) {
    pr[i] = i;
    szz[i] = (int)edge1[i].size();
  }
  sort(qq.begin(), qq.end());
  dfs(0, -1);
  cout << ans << "\n";
  for (int i = 0; i < int(n - 1); i++) {
    a = nu1[i];
    b = nu2[i];
    if (binary_search(qq.begin(), qq.end(), mp(min(a, b), max(a, b)))) {
      unite(a, b);
    } else {
      cout << a + 1 << " " << b + 1 << " ";
      a = get_root(a);
      while (get_root(edge1[a][(int)edge1[a].size() - 1].first) == a) {
        edge1[a].pop_back();
        szz[a]--;
      }
      cout << edge1[a][(int)edge1[a].size() - 1].first + 1 << " "
           << edge1[a][(int)edge1[a].size() - 1].second + 1 << "\n";
      b = edge1[a][(int)edge1[a].size() - 1].first;
      edge1[a].pop_back();
      unite(a, b);
    }
  }
  return 0;
}
