Blackjack

Chef is playing a variant of Blackjack, where $3$ numbers are drawn and each number lies between $1$ and $10$ (with both $1$ and $10$ inclusive). Chef wins the game when the sum of these $3$ numbers is exactly $21$. Given the first two numbers $A$ and $B$, that have been drawn by Chef, what should be $3$-rd number that should be drawn by the Chef in order to win the game? Note that it is possibl