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