#include <bits/stdc++.h>
using namespace std;
const long long int N = 100005;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  {
    long long int i, j, k, n, m, a, b, ans = 0, cnt = 0, sum = 0;
    cin >> n >> m >> a >> b;
    if (n + 8 * a >= m) {
      cout << "0";
      return 0;
    } else if (a > b) {
      long long int x = m - n - (8 * a);
      long long int y = 12 * (a - b);
      ans = (x + y - 1) / y;
      cout << ans << endl;
    } else {
      cout << "-1";
    }
  }
}
