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 n, m, bad[999], w[1000], p[1000], cnt;
vector<int> c;
vector<pair<int, int>> d[1000], br, bu;
void dfs(int v, int p) {
  w[v] = 1;
  for (int i = 0; i < d[v].size(); ++i) {
    if (d[v][i].second == p) continue;
    if (w[d[v][i].second]) {
      if (!bad[d[v][i].first] &&
          !bad[d[v][i].first + (d[v][i].first & 1 ? -1 : 1)]) {
        bad[d[v][i].first] = 1;
        br.push_back(make_pair((d[v][i].first & 1 ? d[v][i].second : v),
                               (d[v][i].first & 1 ? v : d[v][i].second)));
      }
    } else
      dfs(d[v][i].second, v);
  }
}
void dfs2(int v) {
  w[v] = 1;
  for (int i = 0; i < d[v].size(); ++i) {
    if (!w[d[v][i].second]) dfs2(d[v][i].second);
  }
}
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n - 1; ++i) {
    int f, t;
    scanf("%d%d", &f, &t);
    --f;
    --t;
    d[f].push_back(make_pair(2 * i, t));
    d[t].push_back(make_pair(2 * i + 1, f));
  }
  for (int i = 0; i < n; ++i)
    if (!w[i]) dfs(i, -1);
  for (int i = 0; i < n; ++i) w[i] = 0;
  for (int i = 0; i < n; ++i) {
    if (!w[i]) {
      c.push_back(i);
      dfs2(i);
    }
  }
  for (int i = 1; i < c.size(); ++i) bu.push_back(make_pair(c[i], c[i - 1]));
  printf("%d\n", bu.size());
  for (int i = 0; i < bu.size(); ++i)
    printf("%d %d %d %d\n", br[i].first + 1, br[i].second + 1, bu[i].first + 1,
           bu[i].second + 1);
  return 0;
}
