\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning}
\usetikzlibrary{narrow}
\begin{document}
\begin{tikzpicture}[auto, line width=1pt, >=narrow]
\tikzset{block/.style={draw, rectangle, line width=2pt, minimum height=3em, minimum width=3em, outer sep=0pt}}
\tikzset{sumcircle/.style={draw, circle, outer sep=0pt, label=center:{$\sum$}, minimum width=2em}}

\node[block](G1){$G_1$};
\node[block,below=6mm of G1](G2){$G_2$};
\node[sumcircle,left=7mm of G1](sum){};
\node[coordinate,left=13mm of sum](input){};
\node[coordinate,right=13mm of G1](output){};

\draw[->](input)--node[xshift=-2mm]{$R$}(sum);
\draw[->](sum)--(G1);
\draw[->](G1)--node(y)[]{$Y$}(output);
\draw[->](y)|-(G2){};
\draw[->](G2)-|(sum);
\end{tikzpicture}
\end{document}