#include <bits/stdc++.h>
using namespace std;
int h1, h2, a, b, k;
int main() {
  cin >> h1 >> h2 >> a >> b;
  if (h1 + 8 * a < h2 && a <= b) {
    cout << "-1";
    return 0;
  }
  h1 -= 4 * a;
  for (;; k++) {
    h1 += 12 * a;
    if (h1 >= h2) {
      cout << k;
      break;
    }
    h1 -= 12 * b;
  }
  return 0;
}
