#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, k, h;
  cin >> n >> m >> k >> h;
  if ((n + (8 * k)) < m && k <= h) {
    cout << -1 << endl;
    return 0;
  }
  int i = 0;
  while (n < m) {
    if (i == 0)
      n += 8 * k;
    else
      n += 12 * k;
    if (n >= m) break;
    n -= 12 * h;
    i++;
  }
  cout << i << endl;
}
