# bipartite graph with dashed set U

It remains to draw the dashed set U and to place the vertex z downwards. Finally, remains draw the sets $X_1$ and $X_2$ contained in $S-I$. Please how can do this?

    \documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{tikz}
\usetikzlibrary{chains,fit,shapes}
\begin{document}
\begin{tikzpicture}[
node distance = 7mm and 21mm,
start chain = going below,
V/.style = {circle, draw,fill=#1,inner sep=0pt, minimum size=3mm,node contents={}},
every fit/.style = {ellipse, draw=#1, inner ysep=-1mm, inner xsep=5mm},]

\node (y) [V, on chain, label={left:$y$}];
% vertices
\foreach \i in {0,...,3}
{
\node (n1\i) [on chain]{};
\node (n2\i) [above right=0.01mm and 22mm of n1\i]{};
}
\node (z) [V,above right=0.01mm and 22mm of y,label={right:$z$}]{};
% Set I
\node [fit=(n10) (n13)(y),label=above:$I$] {};
% Set S-I
\node [fit=(z)(n20) (n23),label=above:$S-I$] {};

\draw [->,shorten >=1mm, shorten <=1mm](y) edge (z);
\end{tikzpicture}
\end{document}