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;
void dsu_make(int node, vector<int> &pt, vector<int> &ht) {
  pt[node] = node;
  ht[node] = 0;
}
int dsu_find(int node, vector<int> &pt) {
  if (node == pt[node]) return node;
  pt[node] = dsu_find(pt[node], pt);
  return pt[node];
}
void dsu_union(int n1, int n2, vector<int> &pt, vector<int> &ht) {
  n1 = dsu_find(n1, pt);
  n2 = dsu_find(n2, pt);
  if (ht[n1] > ht[n2])
    pt[n2] = n1;
  else
    pt[n1] = n2;
  if (ht[n1] == ht[n2]) ht[n2]++;
}
int main() {
  int n, t1, t2;
  cin >> n;
  vector<int> pt(n, 0), ht(n, 0);
  for (int i = 0; i < n; i++) dsu_make(i, pt, ht);
  vector<pair<int, int>> edge, edge2;
  for (int i = 1; i < n; i++) {
    cin >> t1 >> t2;
    t1--;
    t2--;
    if (dsu_find(t1, pt) == dsu_find(t2, pt))
      edge.push_back(make_pair(t1, t2));
    else
      dsu_union(t1, t2, pt, ht);
  }
  for (int i = 0; i < n; i++) t1 = dsu_find(i, pt);
  set<int> s;
  for (int i = 0; i < n; i++) {
    s.insert(pt[i]);
  }
  while (s.size() > 1) {
    t1 = *(s.begin());
    s.erase(t1);
    t2 = *(s.begin());
    edge2.push_back(make_pair(t1, t2));
  }
  cout << edge.size() << "\n";
  for (int i = 0; i < edge.size(); i++)
    cout << edge[i].first + 1 << " " << edge[i].second + 1 << " "
         << edge2[i].first + 1 << " " << edge2[i].second + 1 << "\n";
  return 0;
}
