documentclass{standalone}
usepackage[T1]{fontenc} %lien codage fonte en sortie
usepackage[latin1]{inputenc} %lien codage d'entrée
usepackage{dejavu}  % fonte en sortie
usepackage{forest}
usepackage{amssymb}
usetikzlibrary{shapes.geometric}
begin{document}
pgfkeys{/forest,
  racine/.append style={minimum width=1cm, fill=white!80,circle,
        tikz={
            draw [circle,draw,line width=0.5pt] ;
            draw [->,line width=1pt] () ++(160:4mm) arc (-200:40:4mm);}
            },
  affecter/.append style={rectangle, minimum height=5mm},
  ecrire/.append style={trapezium, trapezium left angle=110,trapezium right angle=70, minimum height=5mm},
  lire/.append style={trapezium, trapezium left angle=70,trapezium right angle=-70, minimum height=5mm},
  executer/.append style={below=0ex,draw,cloud,aspect=4,cloud puffs=32},
  retourner/.append style={minimum width=2cm, fill=red!40,rectangle},
  nop/.append style={circle},
  sequence/.append style={fill=red!20, rounded corners},
  decision/.append style={diamond, fill=blue!20, text width=5em, inner sep=.5pt, rounded corners},
  si/.append style={fill=red!20, circle},
  tq/.append style={fill=red!20, circle},
  pour/.append style={fill=red!20, circle},
  nuage/.append style={below=0ex,draw,dotted,cloud,aspect=4,cloud puffs=12}
}
begin{forest}
for tree={
  child anchor=north,
  parent anchor=south,
  thick,
  draw,
  align=center
}
[{CalculerSomme1\hline mbox{}\[-1em]VAR entier som,a,b,compteur\} [seq, racine
 [Initialisation, sequence[{a,b $leftarrow$ }, lire][{som$leftarrow$ 0}, affecter ] ]
 [Traitement, sequence
 [POUR, pour
 [ { compteur\de 1\a b\pas 1}, decision]
 [repeter, sequence, edge label={node[midway,left,font=scriptsize]{vrai}}[{som$leftarrow$ som + a}, affecter ] ] ]  ]
 [Resultat, sequence[{ som}, ecrire] ]  ] ]
end{forest}
end{document}