PL/SQL - PROCEDURE IN /OUT PARAMETER

create or replace procedure xx1
(a in out number)
is
begin
     a:=a+4;
end;

OUTPUT :
  declare
      a number:=&a;
begin
     xx1(a);
     dbms_output.put_line('the value is' || a);
end;

  Enter number 6
    the value is10