What is the sum?



Translate the text above into English, please retain the original text's line breaks and format, and output the translation result directly.
$$
\begin{array}{l}
\underbrace{111 \cdots 11}_{1989 \uparrow 1} \times 9=\underbrace{999 \cdots 99}_{1989 \uparrow 9}=10^{1989}-1 \\
\text { Therefore } \underbrace{111 \cdots 11}_{1989 \uparrow 1} \times \underbrace{111 \cdots 11}_{1989 \uparrow 1} \\
=1234567901234 \cdots 012345679 \times\left(10^{1989}-1\right) \\
=1234567901234 \cdots 012345679 \underbrace{000 \cdots 00}_{1989 \uparrow 0}-\underbrace{123456790 \cdots 012345679}_{1988 \times 0} \\
=123456790 \cdots 012345678987654320987654320 \cdots 0987654321 \\
\end{array}
$$

If the last 1 is added to the 8 indicated by the arrow, we get 221 12345679s and 221 98765432s. Therefore, the sum of the digits is $221 \times 81=19,448$.

Answer: The sum of the digits of the product is 19,448.