#include <bits/stdc++.h>
using namespace std;
int main(int argc, char** argv) {
  ios_base::sync_with_stdio(false);
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  b = b - (a + c * 8);
  c = (c - d) * 12;
  if (b <= 0)
    cout << 0 << endl;
  else if (c <= 0)
    cout << -1 << endl;
  else
    cout << (b + c - 1) / c << endl;
  return 0;
}
