#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 10;
void win() {
  int h1, h2, a, b;
  cin >> h1 >> h2 >> a >> b;
  int ans = 0;
  h1 = h1 - 4 * a;
  int k = h1;
  while (h1 < h2) {
    h1 += a * 12;
    if (h1 <= k) {
      cout << -1;
      return;
    }
    k = h1;
    if (h1 >= h2) {
      break;
    }
    h1 -= b * 12;
    ans++;
  }
  cout << ans;
}
int main() {
  int q = 1;
  while (q--) {
    win();
  }
}
