#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5, inf = 0x3f3f3f3f;
int main() {
  int a, b, h1, h2;
  while (cin >> h1 >> h2) {
    cin >> a >> b;
    int add = a * 12, de = b * 12, first = 8 * a;
    if (a <= b) {
      if (h1 + first >= h2)
        puts("0");
      else
        puts("-1");
      continue;
    }
    int ans = 0;
    h1 += first;
    if (h1 >= h2)
      puts("0");
    else {
      add -= de;
      ans += (h2 - h1) / add;
      if ((h2 - h1) % add) ans++;
      printf("%d\n", ans);
    }
  }
  return 0;
}
