\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}H(X|Y)\equiv\sum_{x\in X, y\in Y}-P_Y(y)P_{X|Y=y}(x)\log P_{X|Y=y}(x)\end{equation*}
\end{document}