5. The sum of the digits of the result of the calculation $\underbrace{99 \cdots 99}_{2021 \uparrow 9} \times \underbrace{99 \cdots 99}_{2020 \uparrow 9}$ is $\qquad$
\begin{tabular}{c}
5 \\
\hline 18189
\end{tabular}