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;
template <typename T>
void amax(T& a, T& b) {
  if (a < b) a = b;
}
template <typename T>
void amin(T& a, T& b) {
  if (a > b) a = b;
}
class dsu {
 public:
  long long n;
  vector<long long> Link, Sz;
  dsu(long long N) {
    n = N + 1;
    Sz.resize(n, 1);
    Link.resize(n);
    iota(Link.begin(), Link.end(), 0);
  }
  long long get_par(long long x) {
    while (x != Link[x]) x = Link[x];
    return x;
  }
  bool merge(long long x, long long y) {
    x = get_par(x);
    y = get_par(y);
    if (x == y) return false;
    if (Sz[x] > Sz[y]) swap(x, y);
    Sz[y] += Sz[x];
    Link[x] = y;
    return true;
  }
};
const long long N = 1005;
long long n;
long long vis[N];
vector<long long> adj[N];
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  vector<pair<long long, long long>> stupid;
  dsu d(n);
  for (long long i = 1; i < n; i++) {
    long long x, y;
    cin >> x >> y;
    adj[x].push_back(y);
    adj[y].push_back(x);
    if (d.merge(x, y)) continue;
    stupid.push_back({x, y});
  }
  set<long long> root, node;
  for (long long i = 1; i <= n; i++) {
    long long t = d.get_par(i);
    if (root.count(t) == 0) node.insert(i), root.insert(t);
  }
  cout << stupid.size() << "\n";
  long long prv = -1;
  while (root.size()) {
    if (prv == -1) {
      prv = *root.begin();
      root.erase(prv);
      continue;
    }
    cout << stupid.back().first << " " << stupid.back().second << " ";
    cout << prv << " " << *root.begin() << "\n";
    prv = *root.begin();
    root.erase(prv);
  }
  return 0;
}
