#include <bits/stdc++.h>
int main() {
  int h, h1, h2, c, c1, c2, ans = 0;
  scanf("%d%d", &h1, &h2);
  scanf("%d%d", &c1, &c2);
  c = c1 - c2;
  h = h2 - h1;
  if (c1 <= c2 && c1 * 8 < h) {
    ans = -1;
  } else if (c1 * 8 >= h) {
    ans = 0;
  } else if (c1 > c2) {
    h = h - 20 * c1 + c2 * 12;
    ans++;
    while (h > 0) {
      h -= 12 * c;
      ans++;
    }
  }
  printf("%d", ans);
}
