ERROR: permission denied for sequence cities_id_seq using Postgres

I’m new at postgres (and at database info systems all in all). I ran following sql script on my database:

create table cities (
id serial primary key,
name text not null
);

create table reports (
id serial primary key,
cityid integer not null references cities(id),
reportdate date not null,
reporttext text not null
);

create user www with password 'www';

grant select on cities to www;
grant insert on cities to www;
grant delete on cities to www;

grant select on reports to www;
grant insert on reports to www;
grant delete on reports to www;

grant select on cities_id_seq to www;
grant insert on cities_id_seq to www;
grant delete on cities_id_seq to www;

grant select on reports_id_seq to www;
grant insert on reports_id_seq to www;
grant delete on reports_id_seq to www;

When, as the user www, trying to:

insert into cities (name) values ('London');

I get the following error:

ERROR: permission denied for sequence cities_id_seq

I get that the problem lies with the serial type. That’s why I grant select, insert and delete rights for the *_id_seq to www. Yet this does not fix my problem. What am I missing?