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>
using namespace std;
long long const N = 1e5 + 1;
template <typename T>
class disjointSet {
  T *rank, *parent;
  T n;
  T num;

 public:
  disjointSet(T n) {
    rank = new T[n];
    parent = new T[n];
    this->n = n;
    num = 0;
    makeSet();
  }
  void makeSet() {
    for (T i = 0; i < n; i++) {
      parent[i] = i;
      rank[i] = 0;
      num++;
    }
  }
  T find(T x) {
    if (parent[x] == x) {
      return x;
    }
    parent[x] = find(parent[x]);
    return parent[x];
  }
  bool Union(T x1, T x2) {
    T rep1 = find(x1);
    T rep2 = find(x2);
    if (rep1 == rep2) return 0;
    num--;
    if (rank[rep1] > rank[rep2]) {
      parent[rep2] = rep1;
    } else if (rank[rep1] < rank[rep2]) {
      parent[rep1] = rep2;
    } else {
      rank[rep1] += 1;
      parent[rep2] = rep1;
    }
    return 1;
  }
  T sets() { return num; }
};
void solve() {
  long long n;
  cin >> n;
  disjointSet<long long> dsu(n + 1);
  std::queue<long long> a1, a2, a3;
  for (int i = 0; i < n - 1; i++) {
    long long a, b;
    cin >> a >> b;
    if (dsu.Union(a, b) == 0) {
      a1.push(a);
      a2.push(b);
    }
  }
  for (int i = 2; i <= n; i++) {
    if (dsu.Union(1, i)) {
      a3.push(i);
    }
  }
  cout << a1.size() << "\n";
  while (!a1.empty()) {
    cout << a1.front() << " " << a2.front() << " " << 1 << ' ' << a3.front()
         << "\n";
    a3.pop();
    a2.pop();
    a1.pop();
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
