Heads and Tails

You have three coins in a bag. One is a regulation coin with "heads" on one side and "tails" on the other. The two other coins are counterfeit: one has heads on both sides and the other has tails on both sides. 

Give the bag a good shaking, withdraw one coin at random and lay it on the table without looking at the down face. 

The up face is a heads. What is the probability the down face is tails?