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 double PI = 2 * acos(0.0);
const int Mod = 1E9 + 7;
long long binpow(long long a, long long b, long long m = Mod) {
  a %= m;
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = res * a % m;
    a = a * a % m;
    b >>= 1;
  }
  return res;
}
inline void yes() { std::cout << "Yes\n"; }
inline void no() { std::cout << "No\n"; }
using namespace std;
long long int parent[100005];
long long int siz[100005];
long long int num_com;
long long int find_set(long long int v) {
  if (v == parent[v]) return v;
  return parent[v] = find_set(parent[v]);
}
void make_set(long long int v) {
  parent[v] = v;
  siz[v] = 1;
  num_com = v;
}
vector<pair<long long int, long long int> > com;
void union_sets(long long int a, long long int b) {
  long long int ver1 = a;
  long long int ver2 = b;
  a = find_set(a);
  b = find_set(b);
  if (a == b) com.push_back(make_pair(ver1, ver2));
  if (a != b) {
    num_com--;
    if (siz[a] < siz[b]) swap(a, b);
    parent[b] = a;
    siz[a] += siz[b];
  }
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int n;
  cin >> n;
  for (long long int i = (1); i < (n + 1); i++) make_set(i);
  vector<pair<long long int, long long int> > fre;
  for (long long int i = 0; i < n - 1; i++) {
    long long int x, y;
    cin >> x >> y;
    union_sets(x, y);
  }
  cout << com.size() << '\n';
  map<long long int, long long int> mm;
  for (long long int i = (1); i < (n + 1); i++) {
    mm[parent[i]] = 1;
  }
  long long int last;
  for (auto i : mm) {
    last = i.first;
    break;
  }
  vector<pair<long long int, long long int> > now;
  for (auto i : mm) {
    if (i.first != last) {
      now.push_back(make_pair(last, i.first));
      last = i.first;
    }
  }
  for (long long int i = (0); i < (com.size()); i++) {
    cout << com[i].first << " " << com[i].second << " " << now[i].first << " "
         << now[i].second << '\n';
  }
}
