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;
long long find_set(long long u, long long parent[]) {
  if (u == parent[u]) return u;
  return parent[u] = find_set(parent[u], parent);
}
bool find_union(long long u, long long v, long long parent[],
                long long size[]) {
  u = find_set(u, parent);
  v = find_set(v, parent);
  if (u == v)
    return 1;
  else {
    if (size[u] < size[v]) swap(u, v);
    parent[v] = u;
    size[u] += size[v];
    return 0;
  }
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t;
  t = 1;
  while (t--) {
    long long n;
    cin >> n;
    long long parent[n + 1], size[n + 1];
    for (long long i = 0; i <= n; i++) {
      parent[i] = i;
      size[i] = 1;
    }
    vector<pair<long long, long long> > res1, res2;
    for (long long j = 0; j < n - 1; j++) {
      long long u, v;
      cin >> u >> v;
      if (find_union(u, v, parent, size)) res1.push_back({u, v});
    }
    vector<long long> temp;
    for (long long i = 1; i <= n; i++)
      if (i == parent[i]) temp.push_back(i);
    for (auto i = temp.begin(); i != temp.end() - 1; i++)
      res2.push_back({*i, *(i + 1)});
    long long m = res1.size();
    cout << m << endl;
    for (long long i = 0; i < m; i++)
      cout << res1[i].first << " " << res1[i].second << " " << res2[i].first
           << " " << res2[i].second << endl;
  }
}
