\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
 
\usepackage{tikz}
\usetikzlibrary{arrows,positioning, calc} 
\tikzstyle{vertex}=[draw,fill=black!15,circle,minimum size=18pt,inner sep=0pt]


\begin{document}
\begin{preview}
\begin{tikzpicture}[very thick,level/.style={sibling distance=70mm/#1}]
\node [vertex] (r){$-4$}
  child {
    node [vertex] (a) {$2$}
    child {
      node [vertex] {$5$}
      child {
        node [vertex] {$6$}
        child {node [vertex] {$20$}}
      } 
      child {
        node [vertex] {$9$}
      }
    }
    child {
      node [vertex] {$3$}
      child {node [vertex] {$19$}}
      child {node [vertex] {$7$}}
    }
  }
  child {
    node [vertex] {$-3$}
    child {
      node [vertex] {$8$}
      child {node [vertex] {$17$}}
      child {node [vertex] {$17$}}
    }
    child {
      node [vertex] {$2$}
      child {node [vertex] {$11$}}
      child {node [vertex] {$17$}}
    }
  };
\end{tikzpicture}
\end{preview}
\end{document}