#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  while (cin >> a >> b >> c >> d) {
    if (c <= d) {
      if (a + 8 * c >= b)
        printf("0\n");
      else
        printf("-1\n");
    } else {
      for (int i = 0;; i++) {
        if (a >= b) {
          printf("%d\n", i);
          break;
        }
        a = a + 8 * c;
        if (a >= b) {
          printf("%d\n", i);
          break;
        }
        a = a - 12 * d + 4 * c;
      }
    }
  }
  return 0;
}
