Prevent Primary Key increment when an INSERT statement fail in PostgreSQL

I have a simple table in my PostgreSQL database like this:

CREATE TABLE person_type
(
  id serial NOT NULL,
  name character(55) NOT NULL,
  CONSTRAINT person_type_pkey PRIMARY KEY (id),
  CONSTRAINT person_type_name_key UNIQUE (name)
)

As you can see the id is automatically incremented and the name must be unique. Every time I run an INSERT sql statements, even when it fails, the id is incremented by one.

How can I prevent this? In order that each time a row is inserted successfully, the primary keys are correlative