#include <bits/stdc++.h>
int main() {
  int a, b, c, d;
  scanf("%d%d%d%d", &a, &b, &c, &d);
  b -= a + 8 * c;
  c = (c - d) * 12;
  if (b <= 0)
    printf("0\n");
  else if (c <= 0)
    printf("-1\n");
  else
    printf("%d\n", (b + c - 1) / c);
  return 0;
}
