#include <bits/stdc++.h>
using namespace std;
template <typename T>
class graph {
 public:
  struct edge {
    int from;
    int to;
    T cost;
  };
  vector<edge> edges;
  vector<vector<int>> g;
  int n;
  graph(int _n) : n(_n) { g.resize(n); }
  virtual int add(int from, int to, T cost) = 0;
};
template <typename T>
class forest : public graph<T> {
 public:
  using graph<T>::edges;
  using graph<T>::g;
  using graph<T>::n;
  forest(int _n) : graph<T>(_n) {}
  int add(int from, int to, T cost = 1) {
    assert(0 <= from && from < n && 0 <= to && to < n);
    int id = (int)edges.size();
    assert(id < n - 1);
    g[from].push_back(id);
    g[to].push_back(id);
    edges.push_back({from, to, cost});
    return id;
  }
};
template <typename T>
class dfs_forest : public forest<T> {
 public:
  using forest<T>::edges;
  using forest<T>::g;
  using forest<T>::n;
  vector<int> pv;
  vector<int> pe;
  vector<int> order;
  vector<int> pos;
  vector<int> end;
  vector<int> sz;
  vector<int> root;
  vector<int> depth;
  vector<T> dist;
  dfs_forest(int _n) : forest<T>(_n) {}
  void init() {
    pv = vector<int>(n, -1);
    pe = vector<int>(n, -1);
    order.clear();
    pos = vector<int>(n, -1);
    end = vector<int>(n, -1);
    sz = vector<int>(n, 0);
    root = vector<int>(n, -1);
    depth = vector<int>(n, -1);
    dist = vector<T>(n);
  }
  void clear() {
    pv.clear();
    pe.clear();
    order.clear();
    pos.clear();
    end.clear();
    sz.clear();
    root.clear();
    depth.clear();
    dist.clear();
  }

 private:
  void do_dfs(int v) {
    pos[v] = (int)order.size();
    order.push_back(v);
    sz[v] = 1;
    for (int id : g[v]) {
      if (id == pe[v]) {
        continue;
      }
      auto &e = edges[id];
      int to = e.from ^ e.to ^ v;
      depth[to] = depth[v] + 1;
      dist[to] = dist[v] + e.cost;
      pv[to] = v;
      pe[to] = id;
      root[to] = (root[v] != -1 ? root[v] : to);
      do_dfs(to);
      sz[v] += sz[to];
    }
    end[v] = (int)order.size() - 1;
  }
  void do_dfs_from(int v) {
    depth[v] = 0;
    dist[v] = T{};
    root[v] = v;
    pv[v] = pe[v] = -1;
    do_dfs(v);
  }

 public:
  void dfs(int v, bool clear_order = true) {
    if (pv.empty()) {
      init();
    } else {
      if (clear_order) {
        order.clear();
      }
    }
    do_dfs_from(v);
  }
  void dfs_all() {
    init();
    for (int v = 0; v < n; v++) {
      if (depth[v] == -1) {
        do_dfs_from(v);
      }
    }
    assert((int)order.size() == n);
  }
};
template <typename T>
class lca_forest : public dfs_forest<T> {
 public:
  using dfs_forest<T>::edges;
  using dfs_forest<T>::g;
  using dfs_forest<T>::n;
  using dfs_forest<T>::pv;
  using dfs_forest<T>::pos;
  using dfs_forest<T>::end;
  using dfs_forest<T>::depth;
  int h;
  vector<vector<int>> pr;
  lca_forest(int _n) : dfs_forest<T>(_n) {}
  inline void build_lca() {
    assert(!pv.empty());
    int max_depth = 0;
    for (int i = 0; i < n; i++) {
      max_depth = max(max_depth, depth[i]);
    }
    h = 1;
    while ((1 << h) <= max_depth) {
      h++;
    }
    pr.resize(n);
    for (int i = 0; i < n; i++) {
      pr[i].resize(h);
      pr[i][0] = pv[i];
    }
    for (int j = 1; j < h; j++) {
      for (int i = 0; i < n; i++) {
        pr[i][j] = (pr[i][j - 1] == -1 ? -1 : pr[pr[i][j - 1]][j - 1]);
      }
    }
  }
  inline bool anc(int x, int y) {
    return (pos[x] <= pos[y] && end[y] <= end[x]);
  }
  inline int go_up(int x, int up) {
    assert(!pr.empty());
    up = min(up, (1 << h) - 1);
    for (int j = h - 1; j >= 0; j--) {
      if (up & (1 << j)) {
        x = pr[x][j];
        if (x == -1) {
          break;
        }
      }
    }
    return x;
  }
  inline int lca(int x, int y) {
    assert(!pr.empty());
    if (anc(x, y)) {
      return x;
    }
    if (anc(y, x)) {
      return y;
    }
    for (int j = h - 1; j >= 0; j--) {
      if (pr[x][j] != -1 && !anc(pr[x][j], y)) {
        x = pr[x][j];
      }
    }
    return pr[x][0];
  }
};
template <typename T>
class hld_forest : public lca_forest<T> {
 public:
  using lca_forest<T>::edges;
  using lca_forest<T>::g;
  using lca_forest<T>::n;
  using lca_forest<T>::pv;
  using lca_forest<T>::sz;
  using lca_forest<T>::pos;
  using lca_forest<T>::order;
  using lca_forest<T>::depth;
  using lca_forest<T>::dfs;
  using lca_forest<T>::dfs_all;
  using lca_forest<T>::lca;
  using lca_forest<T>::build_lca;
  vector<int> head;
  vector<int> visited;
  hld_forest(int _n) : lca_forest<T>(_n) { visited.resize(n); }
  void build_hld(const vector<int> &vs) {
    for (int tries = 0; tries < 2; tries++) {
      if (vs.empty()) {
        dfs_all();
      } else {
        order.clear();
        for (int v : vs) {
          dfs(v, false);
        }
        assert((int)order.size() == n);
      }
      if (tries == 1) {
        break;
      }
      for (int i = 0; i < n; i++) {
        if (g[i].empty()) {
          continue;
        }
        int best = -1, bid = 0;
        for (int j = 0; j < (int)g[i].size(); j++) {
          int id = g[i][j];
          int v = edges[id].from ^ edges[id].to ^ i;
          if (pv[v] != i) {
            continue;
          }
          if (sz[v] > best) {
            best = sz[v];
            bid = j;
          }
        }
        swap(g[i][0], g[i][bid]);
      }
    }
    build_lca();
    head.resize(n);
    for (int i = 0; i < n; i++) {
      head[i] = i;
    }
    for (int i = 0; i < n - 1; i++) {
      int x = order[i];
      int y = order[i + 1];
      if (pv[y] == x) {
        head[y] = head[x];
      }
    }
  }
  void build_hld(int v) { build_hld(vector<int>(1, v)); }
  void build_hld_all() { build_hld(vector<int>()); }
  bool apply_on_path(int x, int y, bool with_lca,
                     function<void(int, int, bool)> f) {
    assert(!head.empty());
    int z = lca(x, y);
    if (z == -1) {
      return false;
    }
    {
      int v = x;
      while (v != z) {
        if (depth[head[v]] <= depth[z]) {
          f(pos[z] + 1, pos[v], true);
          break;
        }
        f(pos[head[v]], pos[v], true);
        v = pv[head[v]];
      }
    }
    if (with_lca) {
      f(pos[z], pos[z], false);
    }
    {
      int v = y;
      int cnt_visited = 0;
      while (v != z) {
        if (depth[head[v]] <= depth[z]) {
          f(pos[z] + 1, pos[v], false);
          break;
        }
        visited[cnt_visited++] = v;
        v = pv[head[v]];
      }
      for (int at = cnt_visited - 1; at >= 0; at--) {
        v = visited[at];
        f(pos[head[v]], pos[v], false);
      }
    }
    return true;
  }
};
template <typename T>
class fenwick {
 public:
  vector<T> fenw;
  int n;
  fenwick(int _n) : n(_n) { fenw.resize(n); }
  void modify(int x, T v) {
    while (x < n) {
      fenw[x] += v;
      x |= (x + 1);
    }
  }
  T get(int x) {
    T v{};
    while (x >= 0) {
      v += fenw[x];
      x = (x & (x + 1)) - 1;
    }
    return v;
  }
};
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  hld_forest<int> g(n);
  for (int i = 0; i < n - 1; i++) {
    int x, y;
    cin >> x >> y;
    --x;
    --y;
    g.add(x, y);
  }
  dfs_forest<int> f(n);
  for (int i = 0; i < n - 1; i++) {
    int x, y;
    cin >> x >> y;
    --x;
    --y;
    f.add(x, y);
  }
  g.build_hld(0);
  f.dfs(0);
  cout << n - 1 << '\n';
  fenwick<int> fenw(n);
  for (int i = 1; i < n; i++) {
    fenw.modify(i, 1);
  }
  for (int it = n - 1; it >= 1; it--) {
    int x = f.order[it];
    int y = f.pv[x];
    bool found = false;
    g.apply_on_path(x, y, false, [&](int from, int to, bool up) {
      if (found) {
        return;
      }
      int sum = fenw.get(to) - fenw.get(from - 1);
      if (sum > 0) {
        found = true;
        if (up) {
          int low = from, high = to;
          while (low < high) {
            int mid = (low + high + 1) >> 1;
            sum = fenw.get(to) - fenw.get(mid - 1);
            if (sum > 0) {
              low = mid;
            } else {
              high = mid - 1;
            }
          }
          fenw.modify(low, -1);
          int v = g.order[low];
          cout << v + 1 << " " << g.pv[v] + 1 << " " << x + 1 << " " << y + 1
               << '\n';
        } else {
          int low = from, high = to;
          while (low < high) {
            int mid = (low + high) >> 1;
            sum = fenw.get(mid) - fenw.get(from - 1);
            if (sum > 0) {
              high = mid;
            } else {
              low = mid + 1;
            }
          }
          fenw.modify(low, -1);
          int v = g.order[low];
          cout << v + 1 << " " << g.pv[v] + 1 << " " << x + 1 << " " << y + 1
               << '\n';
        }
      }
    });
    assert(found);
  }
  return 0;
}
