ffss
The bluish background color of the blocks is of course only an example and is easily changed to something else --- or to nothing. Just change or remove the color specifications in the two block styles.
\documentclass{standalone}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning,calc}
\usetikzlibrary{narrow}
\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{blockMinus/.style={draw,minimum width=2em, minimum height=2em,line width=1pt,fill=blue!10,label=center:{$-1$}} }
\tikzset{sumcircle/.style={draw, circle, outer sep=0pt, label=center:{{$\sum$}}, minimum width=2em}}
\begin{document}
\begin{tikzpicture}
\node[block](Adam){Adam};
\node[sumcircle, left=10mm of Adam](sumA){};
\node[block, left = 10mm of sumA](Bertil){Bertil};
\node[sumcircle, left = 10mm of Bertil](sumB){};
\node[block,left=20mm of sumB,yshift=10mm](Cesar){Cesar};
\node[block,below=14mm of Bertil](David){David};
\node[blockMinus,below=6mm of sumB](minusA){};
\draw[->](Cesar.-30)--node{$AA$} ++(12mm,0)|-(sumB);
\draw[->](Cesar.30)--node{$BB$} ++(12mm,0) -|(sumA);
\draw[->](David)--node[swap,xshift=1mm]{$CC$}(David-|minusA)--(minusA);
\draw[->](minusA)--(sumB);
\draw[->](sumB)--(Bertil);
\draw[->](Bertil)--(sumA);
\draw[->](sumA)--node[xshift=-2mm](SS){}(Adam);
\draw[->](SS)|-(David.30);
\draw[->](Adam)--node[xshift=-5](O1){$YY$}++(20mm,0);
\draw[->](O1)|-(David.-30);
\draw[->](Cesar.west) ++(-15mm,0)--node[xshift=-5]{$RR$}(Cesar);
\draw[-,thin](Adam.center) ++(3mm,3mm)--++(8mm,8mm)node[above]{$XX$};
\end{tikzpicture}
\end{document}