@startuml

skinparam backgroundColor transparent

start

if (quality > 0) then (yes)
    :lower quality with 1;
else (no)
endif

:lower sell in with 1;

if (sell in < 0) then (yes)
    if (quality > 0) then (yes)
        :lower quality with 1;
    else (no)
    endif
else (no)
endif

stop

@enduml