# how do I create a border, non-rectangular, surrounding a nodes in tikz?

I have the following MWE.

I would like to draw a border without including the $y_3$ (or s3 in the .tex file) -- so it would be a rectangle with its left upper corner changed so that it bypasses s3. Is there a way to create such a border, basically, a rectangular polygon, surrounding a set of nodes?

(EDIT: I was trying to do it using the last command in the tikz file.)

\documentclass{article}

\usepackage{tikz}

\usetikzlibrary{fit,chains}

\begin{document}

\begin{tikzpicture}[node distance=2cm]

\node(s1) {$y_1$};
\node(s2) [right of = s1] {$y_2$};
\node(s3) [right of = s2]{$y_{3}$};
\node(s4) [right of = s3]{$y_4$};
\node(s5) [right of = s4]{$y_5$};

\draw [->] (s1) -- (s2) ;
\draw [->] (s2) -- (s3) ;
\draw [->] (s3) -- (s4) ;
\draw [->] (s4) -- (s5) ;

\node(x1) [below of = s1]{$x_1$};
\node(x2) [right of = x1] {$X_2$};
\node(x3) [right of = x2] {$X_3$};
\node(x4) [right of = x3] {$X_4$};
\node(x5) [right of = x4] {$X_5$};

\draw [->] (s1) -- (x1) ;
\draw [->] (s2) -- (x2) ;
\draw [->] (s3) -- (x3) ;
\draw [->] (s4) -- (x4) ;
\draw [->] (s5) -- (x5) ;

\node[rectangle,draw=red, fit=(x3) (x4) (x5) (s4) (s5),inner sep=3mm,line width=1mm](rect2) {};

\end{tikzpicture}
\end{document}


If instead of a node you accept a non automatically drawn line ...

\documnentclass{article}

\usepackage{tikz}

\usetikzlibrary{fit,chains, calc}

\begin{document}

\begin{tikzpicture}[node distance=2cm]

\node(s1) {$y_1$};
\node(s2) [right of = s1] {$y_2$};
\node(s3) [right of = s2]{$y_{3}$};
\node(s4) [right of = s3]{$y_4$};
\node(s5) [right of = s4]{$y_5$};

\draw [->] (s1) -- (s2) ;
\draw [->] (s2) -- (s3) ;
\draw [->] (s3) -- (s4) ;
\draw [->] (s4) -- (s5) ;

\node(x1) [below of = s1]{$x_1$};
\node(x2) [right of = x1] {$X_2$};
\node(x3) [right of = x2] {$X_3$};
\node(x4) [right of = x3] {$X_4$};
\node(x5) [right of = x4] {$X_5$};

\draw [->] (s1) -- (x1) ;
\draw [->] (s2) -- (x2) ;
\draw [->] (s3) -- (x3) ;
\draw [->] (s4) -- (x4) ;
\draw [->] (s5) -- (x5) ;

\node[rectangle,draw=red, fit=(x3) (x4) (x5) (s4) (s5),inner sep=3mm,line width=1mm](rect2) {};

\draw[blue, line width=1mm] (x3.south west)-|(s5.north east) --($(s3.north)!0.5!(s4.north)$)|-($(s3.west)!0.5!(x3.west)$)--cycle;
\end{tikzpicture}
\end{document}