There are two players, and each one has a die with six sides from 1 to
6. The probability of each side landing is equal. Now, the two players
roll their dice, and each player only knows the number on his own die. They
will propose prices in turn, until one of them doesn't provide a
higher price. The prices must be positive integers.
The winner will get the money equal to the sum of these
two dice minus the price they provided.

What is the optimal strategy for playing this game?