\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning,calc}
\usetikzlibrary{narrow}
\begin{document}
\tikzset{blockMinus/.style={draw,minimum width=2em, 
    minimum height=2em,line width=1pt,fill=blue!10,
    label=center:{$-1$}}}

\tikzset{every picture/.style={auto, line width=1pt,
     >=narrow,font=\small}}

\tikzset{block/.style={draw, rectangle, line width=2pt,
     fill=blue!10, minimum height=3em, minimum width=3em,
     outer sep=0pt}}

\tikzset{sumcircle/.style={draw, circle, outer sep=0pt,
      label=center:{{$\sum$}}, minimum width=2em}}

\begin{tikzpicture}

\node[block](Adam){Adam};

\node[sumcircle, left=10mm of Adam](sumA){};
\coordinate[left=10 mm of sumA](input){};
\node[block, right=15 mm of Adam](Bertil){Bertil};
\node[block, below=5 mm of Bertil](Cesar){Cesar};
\node[block, below=5 mm of Cesar](David){David};
\node[blockMinus, right=10mm of Cesar](minusA){};
\node[blockMinus](minusB) at ($(Cesar-|sumA)$){};
\node[sumcircle, right=7mm of minusA](sumB){};
\coordinate[right=12mm of Bertil](Bertil1){};
\coordinate[right=12mm of David](David1){};
\coordinate[right=6mm of sumB](out);
\coordinate[below=25mm of out](out2);
\draw[->](input)--node(){$R$}(sumA);
\draw[->](sumA)--(Adam);
\draw[->](Adam)--node(U){$U$}(Bertil);
\draw[->](U)|-(Cesar);
\draw[->](U)|-(David);
\draw[->](Bertil)--node(){$Y$}(Bertil1)--(sumB);
\draw[->](Cesar)--node(){$Y_1$}(minusA);
\draw[->](minusA)--(sumB);
\draw[->](David)--node(){$Y_2$}(David1)--(sumB);
\draw[->](sumB)--(out)--(out2)-|(minusB);
\draw[->](minusB)--(sumA);


\end{tikzpicture}
\end{document}