#include <bits/stdc++.h>
using namespace std;
int x, y, a, b, ans;
int main() {
  scanf("%d%d%d%d", &x, &y, &a, &b);
  if (x + a * 8 < y && a <= b)
    printf("-1");
  else if (y <= x + a * 8)
    printf("0");
  else {
    x = x + a * 8 - b * 12;
    ans++;
    while (x < y) {
      x += a * 12;
      if (x >= y) {
        printf("%d", ans);
        return 0;
      }
      x -= b * 12;
      ans++;
    }
    printf("%d", ans);
  }
  return 0;
}
