97. We call a four-digit number with this property a "central four-digit number": by rearranging the four digits of this number in any order, and arranging all the four-digit numbers (at least 2) formed in ascending order, the original four-digit number is exactly in the middle position. For example, 2021 is a "central four-digit number". Therefore, including 2021, there are $\qquad$ such "central four-digit numbers" in total.
answer: 90