#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int h1, h2, a, b, cnt = 0;
  scanf("%I64d %I64d", &h1, &h2);
  scanf("%I64d %I64d", &a, &b);
  if (a <= b) {
    h1 = (8 * a + h1);
    if (h1 >= h2)
      printf("0\n");
    else
      printf("-1");
    return 0;
  }
  h1 = (8 * a + h1);
  while (h1 <= h2) {
    if (h1 >= h2) break;
    h1 -= (12 * b);
    cnt++;
    h1 += (12 * a);
  }
  printf("%I64d\n", cnt);
  return 0;
}
