#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2, a, b;
  cin >> h1 >> h2 >> a >> b;
  int h = h2 - h1;
  int ans = 0;
  if (h - (8 * a) <= 0) {
    cout << ans << endl;
    return 0;
  }
  h -= (8 * a);
  if (a - b <= 0) {
    cout << -1 << endl;
    return 0;
  }
  int ch = h;
  while (ch > 0) {
    ch -= ((a - b) * 12);
    ans++;
  }
  cout << ans << endl;
  return 0;
}
