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;
int A;
int gcd(int a, int b) {
  if (a < b) swap(a, b);
  if (a % b == 0) return b;
  return gcd(b, a % b);
}
pair<int, int> find(vector<pair<int, int> > &sets, int a) {
  if (sets[a].first != a) sets[a].first = find(sets, sets[a].first).first;
  return sets[a];
}
void unions(vector<pair<int, int> > &sets, int a, int b) {
  pair<int, int> p = find(sets, a);
  pair<int, int> q = find(sets, b);
  if (p.second < q.second) swap(p, q);
  sets[q.first].first = p.first;
  if (p.second == q.second) sets[p.first].second++;
}
void solve() {
  long long int n, i, a, b, x, y, a1, b1;
  cin >> n;
  vector<pair<int, int> > sets(n, {0, 1});
  pair<int, int> p;
  for (i = 0; i < n; i++) sets[i].first = i;
  vector<pair<int, pair<int, int> > > v;
  for (i = 0; i < n - 1; i++) {
    cin >> a >> b;
    a--;
    b--;
    p = find(sets, a);
    a1 = p.first;
    x = p.second;
    p = find(sets, b);
    b1 = p.first;
    y = p.second;
    if (a1 != b1)
      unions(sets, a1, b1);
    else {
      v.push_back({a1, {a + 1, b + 1}});
    }
  }
  set<int> s;
  for (i = 0; i < n; i++) {
    s.insert(sets[i].first);
  }
  auto itr = s.begin();
  x = *itr + 1;
  itr++;
  cout << v.size() << "\n";
  for (i = 0, itr; i < v.size(); i++, itr++)
    cout << v[i].second.first << " " << v[i].second.second << " " << x << " "
         << *itr + 1 << "\n";
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int T = 1;
  while (T--) {
    solve();
  }
  return 0;
}
