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;
#pragma GCC optimize("O3")
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast,unroll-loops,no-stack-protector,fast-math")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
const char nl = '\n';
const long long mod = 1e9 + 7;
const long long N = 1e5 + 5;
vector<long long> parent(N), size1(N);
long long find_set(long long v) {
  if (v == parent[v]) return v;
  return parent[v] = find_set(parent[v]);
}
void make_set(long long v) {
  parent[v] = v;
  size1[v] = 1;
}
void union_sets(long long a, long long b) {
  if (a != b) {
    if (size1[a] < size1[b]) swap(a, b);
    parent[b] = a;
    size1[a] += size1[b];
  }
}
void accio_ac(void) {
  long long n;
  cin >> n;
  for (long long i = 1; i <= n; i++) {
    make_set(i);
  }
  vector<pair<long long, long long> > extra, not_there;
  for (long long i = 1; i < n; i++) {
    long long u, v, x, y;
    cin >> u >> v;
    x = u;
    y = v;
    u = find_set(u);
    v = find_set(v);
    if (u == v) {
      extra.emplace_back(make_pair(x, y));
    } else {
      union_sets(u, v);
    }
  }
  for (long long i = 1; i <= n; i++) {
    for (long long j = i + 1; j <= n; j++) {
      long long a = find_set(i);
      long long b = find_set(j);
      if (a != b) {
        not_there.emplace_back(make_pair(i, j));
        union_sets(a, b);
      }
    }
  }
  long long k = not_there.size();
  cout << k << '\n';
  for (long long i = 0; i < k; i++) {
    cout << extra[i].first << ' ' << extra[i].second << ' '
         << not_there[i].first << ' ' << not_there[i].second << nl;
  }
}
int32_t main(void) {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(10);
  long long __ = 1;
  while (__--) {
    accio_ac();
  }
  return 0;
}
