#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 10;
const long long INF = 1e9 + 19;
template <class T = int>
inline T readInt();
inline double readDouble();
inline int readUInt();
inline int readChar();
inline void readWord(char *s);
inline bool readLine(char *s);
inline bool isEof();
inline int peekChar();
inline bool seekEof();
template <class T>
inline void writeInt(T x, int len);
template <class T>
inline void writeUInt(T x, int len);
template <class T>
inline void writeInt(T x) {
  writeInt(x, -1);
};
template <class T>
inline void writeUInt(T x) {
  writeUInt(x, -1);
};
inline void writeChar(int x);
inline void writeWord(const char *s);
inline void writeDouble(double x, int len = 0);
inline void flush();
static const int buf_size = 4096;
static char buf[buf_size];
static int buf_len = 0, pos = 0;
inline bool isEof() {
  if (pos == buf_len) {
    pos = 0, buf_len = fread(buf, 1, buf_size, stdin);
    if (pos == buf_len) return 1;
  }
  return 0;
}
inline int getChar() { return isEof() ? -1 : buf[pos++]; }
inline int peekChar() { return isEof() ? -1 : buf[pos]; }
inline bool seekEof() {
  int c;
  while ((c = peekChar()) != -1 && c <= 32) pos++;
  return c == -1;
}
inline int readChar() {
  int c = getChar();
  while (c != -1 && c <= 32) c = getChar();
  return c;
}
inline int readUInt() {
  int c = readChar(), x = 0;
  while ('0' <= c && c <= '9') x = x * 10 + c - '0', c = getChar();
  return x;
}
template <class T>
inline T readInt() {
  int s = 1, c = readChar();
  T x = 0;
  if (c == '-') s = -1, c = getChar();
  while ('0' <= c && c <= '9') x = x * 10 + c - '0', c = getChar();
  return s == 1 ? x : -x;
}
inline double readDouble() {
  int s = 1, c = readChar();
  double x = 0;
  if (c == '-') s = -1, c = getChar();
  while ('0' <= c && c <= '9') x = x * 10 + c - '0', c = getChar();
  if (c == '.') {
    c = getChar();
    double coef = 1;
    while ('0' <= c && c <= '9') x += (c - '0') * (coef *= 1e-1), c = getChar();
  }
  return s == 1 ? x : -x;
}
inline void readWord(char *s) {
  int c = readChar();
  while (c > 32) *s++ = c, c = getChar();
  *s = 0;
}
inline bool readLine(char *s) {
  int c = getChar();
  while (c != '\n' && c != -1) *s++ = c, c = getChar();
  *s = 0;
  return c != -1;
}
static int write_pos = 0;
static char write_buf[buf_size];
inline void writeChar(int x) {
  if (write_pos == buf_size)
    fwrite(write_buf, 1, buf_size, stdout), write_pos = 0;
  write_buf[write_pos++] = x;
}
inline void flush() {
  if (write_pos) fwrite(write_buf, 1, write_pos, stdout), write_pos = 0;
}
template <class T>
inline void writeInt(T x, int output_len) {
  if (x < 0) writeChar('-'), x = -x;
  char s[24];
  int n = 0;
  while (x || !n) s[n++] = '0' + x % 10, x /= 10;
  while (n < output_len) s[n++] = '0';
  while (n--) writeChar(s[n]);
}
template <class T>
inline void writeUInt(T x, int output_len) {
  char s[24];
  int n = 0;
  while (x || !n) s[n++] = '0' + x % 10, x /= 10;
  while (n < output_len) s[n++] = '0';
  while (n--) writeChar(s[n]);
}
inline void writeWord(const char *s) {
  while (*s) writeChar(*s++);
}
inline void writeDouble(double x, int output_len) {
  if (x < 0) writeChar('-'), x = -x;
  int t = (int)x;
  writeUInt(t), x -= t;
  writeChar('.');
  for (int i = output_len - 1; i > 0; i--) {
    x *= 10;
    t = std::min(9, (int)x);
    writeChar('0' + t), x -= t;
  }
  x *= 10;
  t = std::min(9, (int)(x + 0.5));
  writeChar('0' + t);
}
int n;
set<int> e[N];
vector<pair<int, int> > g[N];
int p[N];
set<pair<int, int> > needDel;
vector<pair<pair<int, int>, pair<int, int> > > answer;
bool fin[N];
vector<pair<int, int> > e1;
vector<pair<int, int> > e2;
void read() {
  n = readUInt();
  for (int i = 0; i < n - 1; i++) {
    int v, u;
    v = readUInt();
    u = readUInt();
    v--;
    u--;
    e1.push_back(make_pair(v, u));
  }
  for (int i = 0; i < n - 1; i++) {
    int v, u;
    v = readUInt();
    u = readUInt();
    v--;
    u--;
    e2.push_back(make_pair(v, u));
  }
}
int getId(int v) { return (p[v] == v) ? v : p[v] = getId(p[v]); }
void merge(int v, int u) {
  v = getId(v);
  u = getId(u);
  assert(v != u);
  if (g[v].size() < g[u].size()) swap(g[v], g[u]);
  for (; !g[u].empty(); g[u].pop_back()) {
    g[v].push_back(g[u].back());
  }
  p[u] = v;
}
void dfs(int v, int p) {
  for (auto it = e[v].begin(); it != e[v].end();) {
    int u = *it;
    if (u == p || fin[u]) {
      it++;
      continue;
    }
    dfs(u, v);
    int vv = v;
    int uu = u;
    if (vv > uu) swap(vv, uu);
    if (needDel.count(make_pair(vv, uu)) == 1) {
      it++;
      e[v].erase(u);
      e[u].erase(v);
      int top = getId(u);
      assert(top == u);
      bool flag = 0;
      for (int i = (int)g[top].size() - 1; i >= 0; i--) {
        int x = g[top][i].first;
        int y = g[top][i].second;
        auto tmp = g[top].back();
        g[top].pop_back();
        x = getId(x);
        y = getId(y);
        if (x != y) {
          answer.push_back(make_pair(make_pair(v, u), tmp));
          flag = 1;
          if (x != top) swap(x, y);
          assert(x == top);
          if (fin[y]) {
            merge(y, x);
          } else {
            e[y].insert(x);
            e[x].insert(y);
          }
          break;
        }
      }
      assert(flag);
    } else {
      it++;
    }
  }
  for (auto u : e[v]) {
    if (u == p) continue;
    assert(fin[u]);
    merge(v, u);
  }
  fin[v] = 1;
}
void solve() {
  for (auto x : e1) {
    int v = x.first;
    int u = x.second;
    if (v > u) swap(v, u);
    needDel.insert(make_pair(v, u));
    e[v].insert(u);
    e[u].insert(v);
  }
  for (auto x : e2) {
    int v = x.first;
    int u = x.second;
    if (v > u) swap(v, u);
    needDel.erase(make_pair(v, u));
    g[v].push_back(make_pair(v, u));
    g[u].push_back(make_pair(v, u));
  }
  for (int i = 0; i < n; i++) p[i] = i;
  dfs(0, -1);
}
void printTest() {
  printf("%d\n", n);
  for (auto x : e1) printf("%d %d\n", x.first + 1, x.second + 1);
  for (auto x : e2) printf("%d %d\n", x.first + 1, x.second + 1);
}
void stress() {
  for (int tt = 0;; tt++) {
    cerr << "tt"
         << " = " << tt << endl;
    e1.clear();
    e2.clear();
    n = 100;
    for (int i = 0; i < n; i++) e[i].clear();
    for (int i = 0; i < n; i++) g[i].clear();
    needDel.clear();
    answer.clear();
    for (int i = 0; i < n; i++) fin[i] = 0;
    for (int i = 1; i < n; i++) {
      int v = rand() % i;
      int u = rand() % i;
      e1.push_back(make_pair(i, v));
      e2.push_back(make_pair(u, i));
    }
    solve();
  }
}
void printAns() {
  writeInt(answer.size());
  writeChar('\n');
  for (auto x : answer) {
    writeInt(x.first.first + 1);
    writeChar(' ');
    writeInt(x.first.second + 1);
    writeChar(' ');
    writeInt(x.second.first + 1);
    writeChar(' ');
    writeInt(x.second.second + 1);
    writeChar('\n');
  }
  flush();
}
int main() {
  if (1) {
    int k = 1;
    for (int tt = 0; tt < k; tt++) {
      read();
      solve();
      printAns();
    }
  } else {
    stress();
  }
  return 0;
}
