Oracle Forms

I created a code for Login Form:

declare
v_username VARCHAR2(25);
v_passwrd VARCHAR2(25);
a NUMBER;

begin
select username, passwrd
into v_username, v_passwrd
from login
where username = v_username
and passwrd = v_passwrd;

if v_username = :login.username
and v_passwrd = :login.passwrd then
call_form('MENU');
else
a := show_alert('ALERT_INVALID');
end if;

exception
when others then
a := show_alert('ALERT_LOGIN_ERROR');
end;


I want to change it so that only manager's usernames can be valid, everyone else shows a message "Only managers are allowed to login."