NAND perceptron

Adapted from https://texample.net/tikz/examples/nav1d.

\documentclass[tikz,border=10pt]{standalone}
\begin{document}
\tikzset{int/.style  = {draw, circle, fill=blue!20, minimum size=2em}}
\begin{tikzpicture}[auto,>=latex]
  \node [int] (a) {$b=3$};
  \node (b) [below of=a, left of=a, node distance=1cm, coordinate] {a};
  \node (c) [above of=a, left of=a, node distance=1cm, coordinate] {a};
  \node [int] (bnode) [left of=b, distance=0cm] {$x_1$};
  \node [int] (cnode) [left of=c, distance=0cm] {$x_2$};
  \path[->] (bnode) edge node[below] {$-2$} (a);
  \path[->] (cnode) edge node[above] {$-2$} (a);
\end{tikzpicture}
\end{document}