\newcounter{task}
\newcounter{subtask}[task]
\newcommand{\sub}{\refstepcounter{subtask} \par \arabic{subtask}) \ }
\newenvironment{problem}[1][] {
    \refstepcounter{task}
    \ifthenelse{\equal{#1}{}}{}{\setcounter{task}{#1}}
    \textbf{Задача \arabic{task}.}

}{~\\}