Numbers

A non-negative integer is $K$-digit if it can be written using $K$ digits, but K - 1 digits are not enough for this. For example, $43$ is a two-digit number, $2010$ is a four-digit number, and $0$ and $5$ are one-digit numbers.

For given $A$ and $B$, count how many $K$-digit non-negative integers exist, where $K$ is not less than $A$ and not greater than $B$.