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