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