Problem with n to n relation

Hi,

i create n to n relations like this, right?

create table person (
id serial,
name text
);

create table address (
id serial,
street text
...
);

create table person2adress (
id serial,
person_id integer not null references person(id),
address_id integer not null references address(id),
);

than i can select all adresses from one person with id =1 with
select street
from address
where id =
(
select adress_id
from person2adress
where person_id = 1
);

ok so far so good. but you can still insert persons without any
adress. so its a 0..n relation. But how van i achieve that you can´t
insert any person without adress???

thanks in advance
janning