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;
const double pi = 2.0 * acos(0.0);
const double eps = 1.0e-9;
const int dr[] = {-1, 0, 1, 0}, dc[] = {0, 1, 0, -1};
const int dr6[] = {1, 1, 0, -1, -1, 0}, dc6[] = {0, 1, 1, 0, -1, -1};
const int dr8[] = {-1, -1, 0, 1, 1, 1, 0, -1},
          dc8[] = {0, 1, 1, 1, 0, -1, -1, -1};
int n, id[1024], seq, va[1024], vb[1024], vc[1024], vd[1024], k1, k2, rt[1024];
vector<int> g[1024];
int root_of(int x) { return (x == rt[x] ? x : (rt[x] = root_of(rt[x]))); }
void dfs(int i, int f = 0) {
  id[i] = ++seq;
  for (size_t j = g[i].size(); j-- > 0;)
    if (g[i][j] != f) {
      if (id[g[i][j]]) {
        if (id[i] > id[g[i][j]]) va[k1] = i, vb[k1] = g[i][j], ++k1;
      } else
        dfs(g[i][j], i);
    }
}
int main(void) {
  int kase = 1000000000;
  for (int ks = 1; ks <= kase && 1 == scanf("%d", &n); ++ks) {
    for (int i = 1; i <= n; ++i) g[i].clear();
    for (int i = 1, a, b; i < n && 2 == scanf("%d %d", &a, &b); ++i)
      g[a].push_back(b), g[b].push_back(a);
    seq = k1 = k2 = 0;
    memset(id, 0, sizeof(id));
    for (int i = 1; i <= n; ++i)
      if (!id[i]) dfs(i);
    for (int i = 1; i <= n; ++i) rt[i] = i;
    for (int i = 1; i <= n; ++i)
      for (size_t j = g[i].size(); j-- > 0;) {
        int ra = root_of(i), rb = root_of(g[i][j]);
        if (ra != rb) rt[rb] = ra;
      }
    for (int i = 1; i <= n; ++i)
      if (i == root_of(i)) {
        for (int j = i + 1; j <= n; ++j)
          if (j == root_of(j)) vc[k2] = i, vd[k2] = j, ++k2;
        break;
      }
    printf("%d\n", k1);
    for (int i = 0; i < k1; ++i)
      printf("%d %d %d %d\n", va[i], vb[i], vc[i], vd[i]);
  }
  return 0;
}
