text: \documentclass[varwidth=true, border=2pt]{standalone}
  \usepackage{tikz}
  \usetikzlibrary{arrows,positioning}
  \tikzset{
      %Define standard arrow tip
      >=stealth',
      % Define arrow style
      pil/.style={->,thick}
  }
  \tikzstyle{vertex}=[draw,fill=black,circle,minimum size=10pt,inner sep=0pt]
  \begin{document}
    \begin{tikzpicture}
        \node (a)[vertex] at (0,3) {};
        \node (b)[vertex] at (0,1) {};
        \node (c)[vertex] at (1,0) {};
        \node (d)[vertex] at (2,0) {};
        \node (e)[vertex] at (3,0) {};
        \node (f)[vertex] at (4,1) {};
        \node (g)[vertex] at (4,3) {};
  
        \foreach \from/\to in {b/c,c/d,d/e,e/f}
          \draw[line width=2pt] (\from) -- (\to);
  
          \path[line width=2pt] (d) edge[ out=140, in=50
                  , looseness=0.8, loop
                  , distance=2cm]
              node[above=3pt] {$k$} (d);
    \end{tikzpicture}
  \end{document}
  