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>
#pragma GCC optimize("O3")
#pragma GCC target("avx2")
using namespace std;
const double EPS = 0.0000000001;
const long long int mod1 = 998244353;
const long long int mod2 = 1000000007;
const long long int mod3 = 1000000009;
int pred[10000];
int rank1[10000];
void pr(int n) {
  for (int i = 0; i < n; i++) {
    pred[i] = i;
    rank1[i] = 0;
  }
}
int dsu_get(int n) {
  if (pred[n] == n) return n;
  pred[n] = dsu_get(pred[n]);
  return pred[n];
}
void dsu_unite(int a, int b) {
  a = dsu_get(a);
  b = dsu_get(b);
  if (a != b) {
    if (rank1[a] < rank1[b]) swap(a, b);
    pred[b] = a;
    if (rank1[a] == rank1[b]) rank1[a]++;
  }
}
int main() {
  int n;
  cin >> n;
  pr(n);
  vector<pair<int, int> > mass;
  vector<pair<int, int> > res;
  for (int i = 0; i < n - 1; i++) {
    int x, y;
    cin >> x >> y;
    --x;
    --y;
    if (x > y) swap(x, y);
    if (pred[x] != pred[y]) {
      dsu_unite(x, y);
    } else {
      mass.push_back(make_pair(x + 1, y + 1));
    }
  }
  set<int> perm;
  perm.insert(pred[0]);
  for (int i = 1; i < n; i++) {
    if (!perm.count(pred[i])) {
      perm.insert(i);
      res.push_back(make_pair(1, i + 1));
    }
  }
  if (perm.size() == 1 || !mass.size()) {
    cout << 0;
    return 0;
  }
  cout << min(mass.size(), res.size()) << endl;
  for (int i = 0; i < min(mass.size(), res.size()); i++) {
    cout << mass[i].first << " " << mass[i].second << " " << res[i].first << " "
         << res[i].second << endl;
  }
}
