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 int par[2002], siz[2002];
void make(long long int x) {
  par[x] = x;
  siz[x] = 1;
}
long long int findp(long long int x) {
  return x == par[x] ? x : par[x] = findp(par[x]);
}
void combine(long long int a, long long int b) {
  a = findp(a);
  b = findp(b);
  if (a != b) {
    if (siz[a] < siz[b]) swap(a, b);
    par[b] = a;
    siz[a] += siz[b];
  }
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int tt = 1;
  long long int ii = 0;
  while (tt-- && ++ii) {
    long long int n;
    cin >> n;
    for (long long int i = 1; i <= n; i++) make(i);
    vector<pair<long long int, long long int> > ans;
    for (long long int i = 1; i < n; i++) {
      long long int x, y;
      cin >> x >> y;
      long long int xx = findp(x);
      long long int yy = findp(y);
      if (xx == yy)
        ans.push_back({x, y});
      else
        combine(xx, yy);
    }
    set<long long int> s;
    for (long long int i = 1; i <= n; i++) s.insert(par[i]);
    cout << ans.size() << '\n';
    long long int i = 0;
    for (auto x = ++s.begin(); x != s.end() && i < ans.size(); x++)
      cout << ans[i].first << " " << ans[i++].second << " " << *s.begin() << " "
           << *x << '\n';
  }
  return 0;
}
