#include <bits/stdc++.h>
using namespace std;
long long mod = 1e9 + 7;
int main() {
  ios::sync_with_stdio(false);
  int x, y, f, b;
  cin >> x >> y;
  cin >> f >> b;
  if (f * 8 + x >= y) {
    cout << "0\n";
    return 0;
  }
  long long cur = f * 8 + x - 12LL * b, ans = 0;
  while (1) {
    ans++;
    if (cur + 12LL * f >= y || ans > 6e7) break;
    cur = cur + 12LL * f - b * 12LL;
  }
  if (ans >= 6e7)
    cout << "-1\n";
  else
    cout << ans << endl;
  return 0;
}
