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