#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2, a, b;
  cin >> h1 >> h2 >> a >> b;
  int ans = 0;
  bool flag = false;
  int now = h1, next;
  while (!flag) {
    next = now + 8 * a;
    if (next >= h2) {
      flag = 1;
      printf("%d\n", ans);
      return 0;
    }
    ans++;
    now = next;
    next = now - 12 * b;
    now = next;
    next = now + 4 * a;
    now = next;
    if (a <= b) {
      puts("-1");
      return 0;
    }
  }
}
