#include <bits/stdc++.h>
using namespace std;
void masterstroke() {
  long long h1, h2, a, b;
  cin >> h1 >> h2;
  cin >> a >> b;
  if (b >= a) {
    if (a * 8 + h1 >= h2) {
      cout << 0 << endl;
    } else {
      cout << -1 << endl;
    }
  } else {
    long long ht = h1 + 8 * a;
    long long day = 0;
    while (ht < h2) {
      ht -= b * 12;
      ht += a * 12;
      day++;
    }
    cout << day << endl;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long t = 1;
  for (long long i = 0; i < t; i++) masterstroke();
}
