PL/pgSQL block structure

DO $$
DECLARE
  v_message VARCHAR( 255 ) := 'Hello World!';
BEGIN
  raise notice 'hi %',v_message ;
end $$;

do $$
declare
    a integer := 10;
    b integer := 100;
    c integer;
    begin
        if a > b then
         c:= a;
        else
            c:= b;
        end if;
    raise notice ' Maximum number in 10 and 100 : %', c;
    end $$;