#include <bits/stdc++.h>
const int inf = (1 << 30) - 10;
using namespace std;
inline int get_int() {
  int r = 0;
  char c;
  while ((c = getchar()) != ' ' && c != '\n') r = r * 10 + c - '0';
  return r;
}
inline void out(int x) {
  if (x > 10) {
    out(x / 10);
  }
  putchar(x % 10 + '0');
  putchar('\n');
}
int main() {
  int h, H, u, d, l;
  scanf("%d%d%d%d", &h, &H, &u, &d);
  l = 0;
  if (u <= d) {
  }
  while (h < H) {
    h += 8 * u;
    if (h >= H) break;
    l++;
    h -= 12 * d;
    if (u <= d) {
      h += 12 * u;
      if (h < H) {
        l = -1;
        break;
      }
    }
    h += 4 * u;
  }
  printf("%d\n", l);
  return 0;
}
