#include <bits/stdc++.h>
using namespace std;
const long long MAX = 1e5 + 3;
const long long mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    long long h1, h2;
    cin >> h1 >> h2;
    long long a, b;
    cin >> a >> b;
    if (a <= b) {
      if (h2 <= h1 + (8 * a)) {
        cout << "0";
        return 0;
      } else
        cout << "-1";
    } else {
      if (h2 <= h1 + (8 * a)) {
        cout << "0";
        return 0;
      }
      h1 = h1 + (8 * a) - (12 * b);
      long long cnt = 0;
      while (h1 < h2) {
        cnt++;
        h1 = h1 + (12 * a);
        if (h1 >= h2) break;
        h1 = h1 - (12 * b);
      }
      cout << cnt;
    }
  }
  return 0;
}
