#include <bits/stdc++.h>
using namespace std;
int x = 0, a, b, A, B, z;
int main() {
  cin >> B >> A >> a >> b;
  A -= B;
  if (x + 8 * a >= A) {
    printf("0\n");
    return 0;
  }
  if (a <= b) {
    printf("-1\n");
    return 0;
  }
  x += 8 * a;
  x -= 12 * b;
  z = 1;
  while (x + 12 * a < A) {
    x += 12 * (a - b);
    ++z;
  }
  printf("%d\n", z);
  return 0;
}
