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;
const long long mod = 1000000007, INF = 2000000001;
int dx[] = {0, 1, 0, -1, 1, 1, -1, -1, 0},
    dy[] = {-1, 0, 1, 0, -1, 1, 1, -1, 0};
const double EPS = 1e-9;
void make_set(vector<int>& p, int x) { p[x] = x; }
int find_set(vector<int>& p, int x) {
  return p[x] = (p[x] == x) ? x : find_set(p, p[x]);
}
void unite_sets(vector<int>& p, vector<int>& r, int a, int b) {
  int pa = find_set(p, a), pb = find_set(p, b);
  if (r[pa] > r[pb]) swap(pa, pb);
  p[pa] = pb;
  if (r[pa] == r[pb]) ++r[pb];
}
void _a() {}
void _b() {}
void _c() {}
void _d() {
  int n;
  cin >> n;
  vector<int> p(n + 1), r(n + 1, 1);
  for (int i = 1; i <= n; ++i) p[i] = i;
  vector<pair<int, int> > ans;
  while (--n) {
    int a, b;
    cin >> a >> b;
    int pa = find_set(p, a), pb = find_set(p, b);
    if (pa == pb)
      ans.push_back(make_pair(a, b));
    else
      unite_sets(p, r, a, b);
  }
  set<int> s;
  for (int i = 1; i < p.size(); ++i) s.insert(p[i]);
  set<int>::iterator it = s.begin();
  cout << ans.size() << endl;
  for (int i = 0; i < ans.size(); ++i)
    cout << ans[i].first << ' ' << ans[i].second << ' ' << *s.begin() << ' '
         << *++it << endl;
}
void _e() {}
int main() {
  _d();
  return 0;
}
