#include <bits/stdc++.h>
using namespace std;
list<int> adjlist[1005];
bool visited[1005];
int root[1005];
int cmp;
vector<pair<int, int> > v[1005];
int ti = 0;
int arr[1005];
int leader[1005];
int sz[1005];
void init(int n) {
  for (int i = 1; i <= n; i++) {
    leader[i] = i;
    sz[i] = 1;
  }
}
int getleader(int x) {
  int y = x;
  while (leader[y] != y) {
    y = leader[y];
  }
  while (leader[x] != x) {
    int t = x;
    x = leader[x];
    leader[t] = y;
  }
  return y;
}
void uni(int x, int y) {
  if (sz[x] > sz[y]) {
    leader[y] = x;
    sz[x] += sz[y];
  } else {
    leader[x] = y;
    sz[y] += sz[x];
  }
}
void dfs(int s, int p) {
  visited[s] = 1;
  arr[s] = ti++;
  list<int>::iterator it;
  for (it = adjlist[s].begin(); it != adjlist[s].end(); it++) {
    if (visited[*it]) {
      if (*it != p && arr[s] > arr[*it]) {
        v[cmp].push_back(make_pair(s, *it));
      }
    } else
      dfs(*it, s);
  }
  ti++;
}
int main() {
  int n, i, j, k;
  scanf("%d", &n);
  for (i = 0; i < n - 1; i++) {
    scanf("%d %d", &k, &j);
    adjlist[j].push_back(k);
    adjlist[k].push_back(j);
  }
  for (i = 1; i <= n; i++) {
    if (!visited[i]) {
      cmp++;
      root[cmp] = i;
      dfs(i, 0);
    }
  }
  init(cmp);
  printf("%d\n", cmp - 1);
  for (i = 1; i <= cmp; i++) {
    for (j = 0; j < (int)v[i].size(); j++) {
      for (k = 1; k <= cmp; k++) {
        if (getleader(k) != getleader(i)) break;
      }
      printf("%d %d %d %d\n", v[i][j].first, v[i][j].second, v[i][j].first,
             root[k]);
      uni(getleader(k), getleader(i));
    }
  }
  return 0;
}
