Minimal russian LaTeX document

russian.tex

\documentclass{article}
    % General document formatting
    \usepackage[margin=0.5in]{geometry}
    \usepackage[parfill]{parskip}
    \usepackage[utf8]{inputenc}
    \usepackage[T2A]{fontenc}
    \usepackage[english,russian]{babel}
    
    % Related to math
    \usepackage{amsmath,amssymb,amsfonts,amsthm}

\begin{document}

\section*{Введение}

% Таблица
\begin{center}
\begin{tabular}{c | *4{r}}
$ C_n^k $ & 0 & 1 & 2 & 3 \\
 \hline
 0 & 1 & 1 & 1 & 1 \\
 1 & 1 & 2 & 3 & 4 \\
 2 & 1 & 3 & 6 & 10 \\
 3 & 1 & 4 & 10 & 20 \\
\end{tabular}
\end{center}

\end{document}