#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll inf = 1e18;
vector<int> adj[100005], adj1[100005];
ll cnt, ans;
bool vis[100005];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int h1, h2, a, b, diff, av, res;
  cin >> h1 >> h2 >> a >> b;
  diff = h2 - h1;
  if (a <= b) {
    if (8 * a >= diff)
      cout << 0 << '\n';
    else
      cout << -1 << '\n';
  } else {
    av = a - b;
    diff -= (8 * a);
    if (diff <= 0) return cout << 0, 0;
    res = ceil(double(diff) / (12 * av));
    cout << res;
  }
  return 0;
}
