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>
const int INF = 0x3f3f3f3f;
const double PI = acos(-1.0);
const int MAX = 1e5 + 10;
const long long mod = 1e9 + 7;
using namespace std;
int pre[50005];
int Map[1005][1005];
int temp[1005];
struct edge {
  int s;
  int e;
  int flag;
} edge[1005];
int find(int n) {
  int flag = n;
  while (pre[flag] != flag) {
    flag = pre[flag];
  }
  int temp;
  int i = n;
  while (i != flag) {
    temp = pre[i];
    pre[i] = flag;
    i = temp;
  }
  return flag;
}
void join(int x, int y) {
  int fx = find(x);
  int fy = find(y);
  if (fx != fy) {
    pre[max(fx, fy)] = min(fx, fy);
  }
}
int main() {
  int n;
  while (scanf("%d", &n) != EOF) {
    memset(Map, 0, sizeof(Map));
    memset(edge, 0, sizeof(edge));
    for (int i = 1; i <= n; i++) pre[i] = i;
    int u, v;
    int ll = 0;
    for (int i = 1; i < n; i++) {
      scanf("%d%d", &u, &v);
      Map[u][v] = 1;
      if (pre[v] == pre[u]) {
        edge[ll].s = u;
        edge[ll++].e = v;
      }
      join(u, v);
    }
    int sum = 0;
    int rr = 0;
    for (int i = 1; i <= n; i++)
      if (pre[i] == i) {
        sum++;
        temp[rr++] = i;
      }
    if (sum == 1) {
      printf("0\n");
      continue;
    }
    printf("%d\n", sum - 1);
    for (int i = 0; i < rr; i++) {
      for (int j = 0; j < ll; j++) {
        if (!edge[j].flag && edge[j].s != temp[i] && edge[j].e != temp[i]) {
          printf("%d %d %d %d\n", edge[j].s, edge[j].e, edge[j].s, temp[i]);
          edge[j].flag = 1;
        }
      }
    }
  }
  return 0;
}
