
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\usepackage{mathtools}
\pagestyle{empty}
\begin{document}
\[
S_I = \sum_{%
    \substack{(i,j) \in I^2 \\
    i \neq j}} 
    x_i + x_j
\]
\end{document}
