#include <bits/stdc++.h>
using namespace std;
const int Imx = 2147483647;
const long long Lbig = 2e18;
const int mod = 1e9 + 7;
struct fastio {
  char s[100000];
  int it, len;
  fastio() { it = len = 0; }
  inline char get() {
    if (it < len) return s[it++];
    it = 0;
    len = fread(s, 1, 100000, stdin);
    if (len == 0)
      return EOF;
    else
      return s[it++];
  }
  bool notend() {
    char c = get();
    while (c == ' ' || c == '\n') c = get();
    if (it > 0) it--;
    return c != EOF;
  }
} _buff;
inline long long getnum() {
  long long r = 0;
  bool ng = 0;
  char c;
  c = _buff.get();
  while (c != '-' && (c < '0' || c > '9')) c = _buff.get();
  if (c == '-') ng = 1, c = _buff.get();
  while (c >= '0' && c <= '9') r = r * 10 + c - '0', c = _buff.get();
  return ng ? -r : r;
}
template <class T>
inline void putnum(T x) {
  if (x < 0) putchar('-'), x = -x;
  register short a[20] = {}, sz = 0;
  while (x) a[sz++] = x % 10, x /= 10;
  if (sz == 0) putchar('0');
  for (int i = sz - 1; i >= 0; i--) putchar('0' + a[i]);
}
inline char getreal() {
  char c = _buff.get();
  while (c == ' ' || c == '\n') c = _buff.get();
  return c;
}
int n;
int head[500111], nxt[1000111], to[1000111], tot = 1;
int head2[500111], nxt2[1000111], to2[1000111], tot2 = 1;
int f[500111];
bool oc[500111], oc2[500111];
void adde(int x, int y) {
  nxt[++tot] = head[x];
  head[x] = tot;
  to[tot] = y;
}
void adde2(int x, int y) {
  nxt2[++tot2] = head2[x];
  head2[x] = tot2;
  to2[tot2] = y;
}
int gf(int x) { return f[x] == x ? x : f[x] = gf(f[x]); }
map<pair<int, int>, int> mpid;
int nid[500111];
void dfs(int x, int p = -1) {
  for (int i = head[x]; i; i = nxt[i]) {
    int u = to[i];
    if (u == p) continue;
    dfs(u, x);
    if (oc[i >> 1]) continue;
    int tmp = nid[gf(u)];
    if (!tmp) continue;
    putnum(u), putchar(' '), putnum(x), putchar(' ');
    putnum(to2[tmp]), putchar(' '), putnum(to2[tmp ^ 1]), putchar('\n');
  }
}
void dfs2(int x, int p = -1) {
  for (int i = head2[x]; i; i = nxt2[i]) {
    int u = to2[i];
    if (u == p) continue;
    if (!oc2[i >> 1]) nid[gf(u)] = i;
    dfs2(u, x);
  }
}
int main() {
  n = getnum();
  for (int i = 1; i <= n; i++) f[i] = i;
  for (int i = 1; i < n; i++) {
    int x, y;
    x = getnum(), y = getnum();
    if (x > y) swap(x, y);
    mpid[make_pair(x, y)] = i;
    adde(x, y);
    adde(y, x);
  }
  int cnt = n - 1;
  for (int i = 1; i < n; i++) {
    int x, y;
    x = getnum(), y = getnum();
    if (x > y) swap(x, y);
    if (mpid.find(make_pair(x, y)) != mpid.end()) {
      oc[mpid[make_pair(x, y)]] = 1;
      oc2[i] = 1;
      cnt--;
      f[gf(x)] = gf(y);
    }
    adde2(x, y);
    adde2(y, x);
  }
  dfs2(1);
  putnum(cnt), putchar('\n');
  dfs(1);
  return 0;
}
