\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[russian]{babel}
\begin{document}

\newcommand{\CustomSum}[2]{\sum_{i=1}^{#1} (#2)^i}

$$A = \CustomSum{N+M}{a+b} - \CustomSum{N}{a-b} - \CustomSum{M}{b-a}$$

\end{document}