#include <bits/stdc++.h>
using namespace std;
int a, b, h1, h2, t, h;
int main() {
  while (~scanf("%d%d%d%d", &h1, &h2, &a, &b)) {
    if (8 * a + h1 >= h2) {
      printf("0\n");
      continue;
    }
    if (a <= b) {
      if (8 * a + h1 - 12 * b + 12 * a >= h2)
        printf("1\n");
      else
        printf("-1\n");
      continue;
    }
    h = 8 * a + h1 - 12 * b;
    t = 1;
    while (true) {
      h += 12 * a;
      if (h >= h2) break;
      ++t;
      h -= 12 * b;
    }
    printf("%d\n", t);
  }
  return 0;
}
