4
1 4
3 1
3 4
 5
4 1
4 3
5 3
2 4
 21
7 15
13 1
14 3
4 10
2 3
16 18
17 20
16 20
8 4
3 12
2 17
13 11
16 1
13 2
13 5
8 9
6 14
3 17
16 9
13 8
 3
3 1
3 2
 6
5 2
5 3
1 4
3 1
5 6
 60
17 34
46 22
50 44
46 33
41 21
31 33
48 6
38 19
35 60
2 24
49 29
7 53
34 1
19 55
32 1
31 42
27 28
4 53
6 50
21 34
1 10
12 36
54 8
16 13
51 43
45 51
54 20
13 53
34 33
49 33
51 11
59 34
15 5
59 28
30 39
13 30
58 4
34 14
3 9
19 34
4 18
26 48
56 54
3 43
57 25
3 41
35 3
48 44
19 13
54 1
23 31
59 47
5 1
46 40
6 26
20 25
37 5
17 24
20 52
 3
3 2
1 2
 39
6 13
15 39
10 35
31 28
4 21
12 39
3 7
3 13
6 1
5 14
36 28
12 15
18 38
30 29
19 34
36 16
20 22
8 13
38 32
26 39
21 37
1 7
15 27
12 26
8 3
6 14
29 2
25 23
32 21
5 16
32 25
6 8
13 10
23 30
34 37
29 33
28 14
36 5
 10
5 9
8 5
7 6
7 9
3 9
2 1
7 2
3 6
7 1

1
3 4 1 2  0
 3
13 2 1 7 3 17 1 19 13 8 1 21  0
 0
 2
48 44 1 6 6 26 1 12  0
 7
12 15 1 2 1 7 1 9 12 26 1 11 8 3 1 12 6 8 1 17 28 14 1 20 36 5 1 24  2
3 6 1 4 7 1 1 10 
#include <bits/stdc++.h>
#pragma warning(disable : 4996)
struct Edge {
  unsigned long a, b, x, y;
  Edge() {}
  Edge(const unsigned long _a, const unsigned long _b, const unsigned long _x,
       const unsigned long _y)
      : a(_a), b(_b), x(_x), y(_y) {}
};
bool FindNonZero(const std::vector<unsigned long>& be, unsigned long& idx) {
  bool bUnvisitedFound = false;
  idx = 0;
  for (unsigned long i = 0; i < be.size() && !bUnvisitedFound; i++) {
    if (!be[i]) {
      idx = i;
      bUnvisitedFound = true;
    }
  }
  return bUnvisitedFound;
}
bool FindLoop(std::vector<std::set<unsigned long> >& links,
              std::vector<unsigned long>& be, const unsigned long idx,
              unsigned long& loopA, unsigned long& loopB) {
  const std::set<unsigned long>& link = links[idx];
  be[idx] = true;
  loopA = idx;
  for (std::set<unsigned long>::const_iterator p = link.begin();
       p != link.end(); ++p) {
    const unsigned long next = *p;
    if (be[next]) {
      loopA = idx;
      loopB = next;
      return true;
    } else {
      links[idx].erase(next);
      links[next].erase(idx);
      return FindLoop(links, be, next, loopA, loopB);
    }
  }
  return false;
}
void MarkComp(const std::vector<std::set<unsigned long> >& links,
              std::vector<unsigned long>& be, const unsigned long idx) {
  const std::set<unsigned long>& link = links[idx];
  be[idx] = true;
  for (std::set<unsigned long>::const_iterator p = link.begin();
       p != link.end(); p++) {
    const unsigned long next = *p;
    if (!be[next]) {
      MarkComp(links, be, next);
    }
  }
}
int main() {
  size_t n;
  std::cin >> n;
  std::vector<std::set<unsigned long> > links(n);
  for (size_t i = 0; i < n - 1; i++) {
    unsigned long a, b;
    std::cin >> a >> b;
    a--;
    b--;
    links[a].insert(b);
    links[b].insert(a);
  }
  std::vector<Edge> ans;
  bool bFinished = false;
  while (!bFinished) {
    std::vector<unsigned long> be(n);
    unsigned long loopA = 0, loopB = 0;
    std::vector<std::set<unsigned long> > links_tmp(links);
    if (FindLoop(links_tmp, be, 0, loopA, loopB)) {
      std::vector<unsigned long>(n).swap(be);
      MarkComp(links, be, loopA);
      unsigned long nonzeroIdx = 0;
      if (FindNonZero(be, nonzeroIdx)) {
        links[loopA].erase(loopB);
        links[loopB].erase(loopA);
        links[loopA].insert(nonzeroIdx);
        links[nonzeroIdx].insert(loopA);
        ans.push_back(Edge(loopA, loopB, nonzeroIdx, loopA));
      }
    } else {
      bFinished = true;
    }
  }
  std::cout << ans.size() << std::endl;
  for (size_t i = 0; i < ans.size(); i++) {
    const Edge& edge = ans[i];
    std::cout << (edge.a + 1) << ' ';
    std::cout << (edge.b + 1) << ' ';
    std::cout << (edge.x + 1) << ' ';
    std::cout << (edge.y + 1) << std::endl;
  }
}
