#include <bits/stdc++.h>
using namespace std;
int h1, h2, a, b, mor, nigh, day;
int main() {
  while (cin >> h1 >> h2) {
    cin >> a >> b;
    if (a <= b) {
      if (8 * a + h1 >= h2)
        cout << "0\n";
      else
        cout << "-1\n";
      continue;
    }
    bool flag = 1;
    day = 0;
    while (1) {
      if (flag) {
        nigh = h1 + 8 * a;
        if (nigh >= h2) {
          cout << "0\n";
          break;
        }
        day++;
        flag = 0;
        continue;
      }
      mor = (nigh - 12 * b) ? (nigh - 12 * b) : 0;
      nigh = mor + 12 * a;
      if (nigh >= h2) {
        cout << day << endl;
        break;
      }
      day++;
    }
  }
  return 0;
}
