#include <bits/stdc++.h>
inline long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
inline void sub(long long &a, long long b) {
  a -= b;
  if (a < 0) a += 1000000007;
}
inline void add(long long &a, long long b) {
  a += b;
  if (a >= 1000000007) a -= 1000000007;
}
template <typename T>
inline T const &MAX(T const &a, T const &b) {
  return a > b ? a : b;
}
template <typename T>
inline T const &MIN(T const &a, T const &b) {
  return a < b ? a : b;
}
inline long long qp(long long a, long long b) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = ans * a % 1000000007;
    a = a * a % 1000000007, b >>= 1;
  }
  return ans;
}
inline long long qp(long long a, long long b, long long c) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = ans * a % c;
    a = a * a % c, b >>= 1;
  }
  return ans;
}
using namespace std;
const unsigned long long ba = 233;
const double eps = 1e-8;
const long long INF = 0x3f3f3f3f3f3f3f3f;
const int N = 10000 + 10, maxn = 10000000 + 10, inf = 0x3f3f3f3f;
int main() {
  int h1, h2, a, b;
  scanf("%d%d%d%d", &h1, &h2, &a, &b);
  if (h1 + 8 * a >= h2)
    puts("0");
  else {
    if (a <= b)
      puts("-1");
    else {
      h1 += 8 * a - 12 * b;
      int ans = 1;
      while (h1 + 12 * a < h2) h1 += 12 * (a - b), ans++;
      printf("%d\n", ans);
    }
  }
  return 0;
}
