#include <bits/stdc++.h>
using namespace std;
typedef int *arrint;
const int maxn = 2e3 + 55;
int main() {
  int h1, h2, a, b, i, j, k;
  scanf("%d", &h1), scanf("%d", &h2);
  scanf("%d", &a), scanf("%d", &b);
  k = 0;
  i = h1;
  j = 1;
  while (i < h2) {
    if (j == 1) {
      i = i + a * 8;
      j = 0;
      if (a <= b && i < h2) {
        k = -1;
        break;
      }
    } else {
      i = i + a * 12;
    }
    if (i >= h2) {
      break;
    } else {
      k++;
      i -= b * 12;
    }
  }
  printf("%d", k), printf("%c", '\n');
  return 0;
}
