#include <bits/stdc++.h>
using namespace std;
const int N = 1006;
struct Edge {
  int s, t, next;
  bool flag;
  void init(int a, int b, int c) { s = a, t = b, next = c, flag = false; }
} edge[N * 2];
int head[N], tot;
void add_edge(int s, int t) {
  edge[tot].init(s, t, head[s]);
  head[s] = tot++;
  edge[tot].init(t, s, head[t]);
  head[t] = tot++;
}
int n, cnt[N], fa[N];
bool vs[N];
void DFS(int u, int pre) {
  vs[u] = 1;
  for (int e = head[u]; e != -1; e = edge[e].next) {
    int v = edge[e].t;
    if (v == pre || vs[v]) continue;
    edge[e].flag = 1, edge[e ^ 1].flag = 1;
    DFS(v, u);
  }
}
int find_fa(int x) {
  if (fa[x] != x) fa[x] = find_fa(fa[x]);
  return fa[x];
}
int main() {
  scanf("%d", &n);
  memset(head, -1, sizeof head);
  tot = 0;
  for (int i = 1; i <= n; i++) fa[i] = i;
  for (int i = 1, a, b; i < n; i++) {
    scanf("%d%d", &a, &b);
    add_edge(a, b);
    int f1 = find_fa(a);
    int f2 = find_fa(b);
    if (f1 != f2) fa[f1] = f2;
  }
  memset(vs, 0, sizeof vs);
  for (int i = 1; i <= n; i++)
    if (!vs[i]) DFS(i, -1);
  memset(cnt, 0, sizeof cnt);
  int ans = 0;
  for (int i = 0; i < tot; i += 2)
    if (!edge[i].flag) {
      ans++;
      cnt[find_fa(edge[i].s)]++;
      cnt[find_fa(edge[i].t)]++;
    }
  printf("%d\n", ans);
  for (int e = 0; e < tot; e += 2) {
    if (edge[e].flag) continue;
    printf("%d %d ", edge[e].s, edge[e].t);
    for (int i = 1; i <= n; i++) {
      if (cnt[find_fa(i)] != 0 || find_fa(i) == find_fa(edge[e].s)) continue;
      printf("%d %d\n", edge[e].s, i);
      fa[find_fa(i)] = find_fa(edge[e].s);
      break;
    }
    cnt[find_fa(edge[e].s)]--;
    cnt[find_fa(edge[e].t)]--;
  }
  return 0;
}
