#include <bits/stdc++.h>
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
const int inf = 0x3f3f3f3f;
inline long long read() {
  register long long x = 0, f = 1;
  register char c = getchar();
  for (; !isdigit(c); c = getchar())
    if (c == '-') f = -1;
  for (; isdigit(c); c = getchar()) x = (x << 1) + (x << 3) + (c ^ 48);
  return x * f;
}
void write(long long x) {
  if (x < 0) x = -x, putchar('-');
  if (x >= 10) write(x / 10);
  putchar(x % 10 + '0');
}
void writeln(long long x) {
  write(x);
  puts("");
}
const int N = 505050;
struct Ans {
  int a, b, c, d;
} ans[N];
int n, x, y, cnt, f[N], fa[2][N];
vector<int> e[2][N];
int get(int x) { return f[x] == x ? x : f[x] = get(f[x]); }
void dfs(int id, int x, int f) {
  for (int i = 0; i < e[id][x].size(); i++)
    if (e[id][x][i] != f) {
      fa[id][e[id][x][i]] = x;
      dfs(id, e[id][x][i], x);
    }
}
void dfs2(int x) {
  for (int i = 0; i < e[0][x].size(); i++)
    if (e[0][x][i] != fa[0][x]) {
      int u = e[0][x][i];
      dfs2(u);
      if (u != fa[1][x] && x != fa[1][u])
        ans[++cnt] = (Ans){u, x, get(u), fa[1][get(u)]};
    }
}
int main() {
  scanf("%d", &n);
  for (int id = 0; id < 2; id++)
    for (int i = 1; i < n; i++) {
      scanf("%d%d", &x, &y);
      e[id][x].push_back(y);
      e[id][y].push_back(x);
    }
  dfs(0, 1, 0);
  dfs(1, 1, 0);
  f[1] = 1;
  for (int i = 2; i <= n; i++) {
    int u = fa[1][i];
    if (fa[0][i] == u || fa[0][u] == i)
      f[i] = u;
    else
      f[i] = i;
  }
  dfs2(1);
  printf("%d\n", cnt);
  for (int i = 1; i <= cnt; i++)
    printf("%d %d %d %d\n", ans[i].a, ans[i].b, ans[i].c, ans[i].d);
}
