Bust at Ten

An arcade wheel has $8$ equal sectors numbered $1$ through $8$. You spin once and see the result $f$. You may cash out $f$ dollars, or spin a second time. If the second spin shows $s$, you collect $f + s$ dollars when $f + s \le 9$, but you bust and collect nothing when $f + s \ge 10$. For which first spins should you spin again, and what is the game worth played optimally?