\begin{center}
\begin{tikzpicture}[scale=0.2]
\tikzstyle{every node}+=[inner sep=0pt]
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\draw (11,-26.9) node {$s_2$};
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\draw (23.6,-15) node {$s_1$};
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\draw (23.6,-38) node {$s_4$};
\draw [black] (36.7,-26.9) circle (3);
\draw (36.7,-26.9) node {$s_3$};
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\end{center}