#include <bits/stdc++.h>
using namespace std;
bool f(int x, int y) { return x > y; }
int main() {
  int a, b, h1, h2, dia, z;
  scanf("%d %d", &h1, &h2);
  scanf("%d %d", &a, &b);
  dia = 0;
  z = h1;
  while (h1 < h2) {
    h1 += 8 * a;
    if (h1 >= h2) break;
    h1 -= 12 * b;
    dia++;
    h1 += 4 * a;
    if (h1 <= z) {
      printf("-1\n");
      return 0;
    }
  }
  printf("%d\n", dia);
  return 0;
}
