Kaidanov Tzvi Gregory Technical Blog

Very simple .
You can decrease the number of your current sequence .
Important to set the nextval, so that the new value will be caught in the sequence.
And most important to change back the increment by 1 , otherwise it will decrease each time you"ll activate the sequence by the negative number you've defined in the first row.

alter sequence customer_seq increment by -40;
select customer_seq.nextval from dual;
alter sequence customer_seq increment by 1;