11. The blackboard is written with 1989 consecutive natural numbers $1, 2, 3, \ldots, 1989$. First, perform the following transformation: erase any two numbers on the blackboard, and add the remainder obtained by dividing the sum of the erased two numbers by 19. After several transformations, there are two numbers left on the blackboard, one of which is 89, and the other is a single-digit number. This single-digit number is ( ).