5_5

\documentclass{standalone}
\usepackage[T1]{fontenc}
\usepackage[latin2]{inputenc}
\usepackage[english]{babel}
\usepackage{tikz}
\usetikzlibrary{calc,through,backgrounds,positioning,fit}
\usetikzlibrary{shapes,arrows,shadows}

\begin{document}

\begin{tikzpicture}[auto,
decision/.style = {diamond, draw=blue, thick, fill=blue!20, text width=1.5cm,
                   align=flush center, inner sep=1pt},
block/.style = {rectangle, draw=blue, thick, fill=blue!20, text width=2cm,
                align=center, minimum height=1.3cm},
start/.style = {draw=red, thick, ellipse, fill=red!20, minimum height=8mm},
inout/.style = {draw=green!60!black, thick, fill=green!20, trapezium, trapezium left angle=80,
                trapezium right angle=-80, minimum height=10mm, text width=1cm, align=center,
                inner sep=1pt},
line/.style ={draw, thick, -latex}]

\matrix[column sep=5mm,row sep=7mm]
{
% row 1
&&& \node [start] (start) {start}; &&& \\
% row 2
&&& \node [inout] (read) {read($a, b$)}; &&& \\

% row 4
&&& \node [decision] (decide1) {$a = b$}; &&& \\
% row 5
&&\node [decision] (decide) {$a < b$}; &&&
&&&\node [inout] (write) {write($a$)}; \\
% row 6
\node[block] (update1) {$a = a-b$};&&&
\node[block] (update) {$b = b-a$};&&&
&&\node [start] (stop) {stop}; \\
% row
};

\begin{scope}[every path/.style=line]
\path(start) -- (read);
\path(read) -- (decide1);
\path(decide1)  -| node [near start] {$no$} (decide);
\path(decide1)  -| node [near start] {$yes$} (write);
\path(decide)  -| node [near start] {$no$} (update1);
\path(decide)  -| node [near start] {$yes$} (update);
\path(write) -- (stop);
\path (update1.south) |- ++(-1.5,-1) coordinate(A) |- ($ (decide1.north) + (0,0.4) $) coordinate(B);
\path (update.south) |- (A) |- (B);
\end{scope}
\end{tikzpicture}
\end{document}