#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  long long int h1, h2;
  cin >> h1 >> h2;
  long long int a, b;
  cin >> a >> b;
  long long int start = 0;
  long long int step = 1;
  long long int height = h2 - h1;
  long long int current = 0;
  long long int day = 0;
  long long int count = 0;
  long long int cool = 1;
  while (cool) {
    current += (8 * a);
    if (current >= height) {
      break;
    }
    current -= (12 * b);
    day++;
    current += (4 * a);
    if (current >= height) {
      break;
    }
    count++;
    if (count >= 1000100) {
      printf("-1");
      return 0;
    }
  }
  cout << day;
  return 0;
}
