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