#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2;
  int a, b;
  scanf("%d%d", &h1, &h2);
  scanf("%d%d", &a, &b);
  if (a <= b && h1 + a * 8 < h2) {
    printf("-1\n");
  } else {
    int cnt = 0;
    int now = h1 - 4 * a;
    while (now + a * 12 < h2) {
      now += a * 12;
      now -= b * 12;
      cnt++;
    }
    printf("%d\n", cnt);
  }
  return 0;
}
