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;
vector<vector<int> > g;
vector<pair<pair<int, int>, pair<int, int> > > res;
int was[1010], was1[1010];
int k = -1;
int comp[1010];
int d[1010][1010];
int n;
void dfs(int v) {
  if (was[v] != -1) return;
  was[v] = k;
  for (int i = (0); i < (g[v].size()); ++i) dfs(g[v][i]);
}
void dfs1(int v1, int v2) {
  if (was1[v1] == 0) {
    if ((d[v2][v1] == 1) && (d[v1][v2] == 1))
      return;
    else {
      res.push_back(make_pair(make_pair(v2, v1), make_pair(0, 0)));
      return;
    }
  }
  if (was1[v1] == 1) return;
  was1[v1] = 0;
  for (int i = (0); i < (g[v1].size()); ++i) {
    d[v1][g[v1][i]] = 1;
    dfs1(g[v1][i], v1);
  }
  was1[v1] = 1;
}
int main() {
  scanf("%d", &n);
  g.resize(n);
  memset(was, -1, sizeof(was));
  memset(comp, -1, sizeof(comp));
  memset(d, 0, sizeof(d));
  for (int i = (0); i < (n - 1); ++i) {
    int a1, b1;
    scanf("%d%d", &a1, &b1);
    g[a1 - 1].push_back(b1 - 1);
    g[b1 - 1].push_back(a1 - 1);
  }
  for (int i = (0); i < (n); ++i) {
    if (was[i] == -1) {
      k++;
      comp[k] = i;
      dfs(i);
    }
  }
  memset(was1, -1, sizeof(was1));
  for (int i = (0); i < (n); ++i)
    if (was1[i] == -1) dfs1(i, -1);
  for (int i = (0); i < (res.size()); ++i) {
    res[i].second.first = comp[i];
    res[i].second.second = comp[i + 1];
  }
  printf("%d\n", res.size());
  for (int i = (0); i < (res.size()); ++i)
    printf("%d %d %d %d\n", res[i].first.first + 1, res[i].first.second + 1,
           res[i].second.first + 1, res[i].second.second + 1);
  return 0;
}
