#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long h1, h2;
  cin >> h1 >> h2;
  long a, b;
  cin >> a >> b;
  if (h2 - h1 <= 8 * a)
    cout << 0;
  else if (b >= a) {
    cout << -1;
    return 0;
  } else {
    h1 += 8 * a;
    long count = 0;
    while (h1 < h2) {
      h1 -= 12 * b;
      h1 += 12 * a;
      count++;
    }
    cout << count;
  }
}
