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;
int a, b, c, d, e, pi, pas[1009], z, n[1009], ni, o[1009], zk;
vector<int> v[1009], g[1009], p[1009];
vector<pair<int, int> > l[1009];
map<pair<int, int>, bool> mm, mn;
int bo[1009];
bool m[1009];
void dfs(int q, int w) {
  if (bo[q] != 0) {
    if (mn[make_pair(q, w)] == 1) return;
    mn[make_pair(q, w)] = 1;
    mn[make_pair(w, q)] = 1;
    l[pi].push_back(make_pair(q, w));
    return;
  }
  bo[q] = 1;
  for (vector<int>::iterator it = v[q].begin(); it != v[q].end(); it++) {
    if (w == (*it)) continue;
    dfs((*it), q);
  }
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  cin >> a;
  for (b = 1; b < a; b++) {
    cin >> c >> d;
    v[c].push_back(d);
    v[d].push_back(c);
  }
  for (b = 1; b <= a; b++) {
    if (bo[b] != 0) continue;
    pi++;
    o[pi] = b;
    dfs(b, 0);
  }
  cout << pi - 1 << endl;
  m[1] = 1;
  zk = 0;
  for (b = 1; b <= pi; b++) {
    d = 0;
    for (c = 1; c <= pi; c++) {
      if (c == b) continue;
      if (l[b].size() == 0) break;
      if (mm[make_pair(b, c)] == 1) continue;
      mm[make_pair(b, c)] = 1;
      mm[make_pair(c, b)] = 1;
      if (pi - 1 == zk) return 0;
      zk++;
      cout << l[b][d].first << " " << l[b][d].second << " " << l[b][d].second
           << " " << o[c] << endl;
      d++;
      if (l[b].size() == d + 1) continue;
    }
  }
  return 0;
}
