Clairvoyance Across Three States

The state is High, Mid, or Low with probabilities $\frac12,\frac13,\frac16$. You pick action Long or Flat once. Long pays $12,\ -3,\ -9$ in High, Mid, Low respectively; Flat pays $0$ in every state. A clairvoyant will tell you the exact state before you choose. What is the difference between your expected payoff acting on the clairvoyant's report and your expected payoff using the single best action chosen in advance?