89. The blackboard is written with a number $\underbrace{222 \cdots 2}_{9 \uparrow 2}$, and the following operation is performed: erase the last digit, multiply the number on the blackboard by 4, and then add the last digit that was just erased. Continue performing such operations until a single-digit number appears on the blackboard, then this number is $\qquad$.
answer: 6