# PSTricks plots wrong

I want to plot a parabola and use therefore the following code:

\usepackage{pst-plot}
\usepackage{pst-math}
%...
\begin{pspicture}(-2,-3.5)(6,3)
\psgrid[griddots=10,gridlabels=0pt,subgriddiv=0,gridcolor=black!20]
\psaxes(0,0)(-2,-3.5)(6,3)
\psset{algebraic,linewidth=1.5pt}
\psplot[linecolor=red]{-1}{5}{-0.5(x-2)^2+2}
\end{pspicture}


But I get this wrong function plotted. I can't find my mistake. What is wrong?

\psplot[linecolor=red]{-1}{5}{-0.5*(x-2)^2+2}