One question to two door guards, liar and truth teller

Two doors face you: one leads to a job offer and the other to the exit. Each door has a guard; one guard always lies and the other always tells the truth, but you do not know which is which. You may ask a single guard one yes/no question. What should you ask to be sure of reaching the offer?

Pick a guard and ask: "If I asked the other guard whether your door leads to the offer, would he answer yes?" The reply passes through exactly one liar, so it is always false. On yes take the other door; on no take this guard's door.