Hi,
To achieve that, you need to first set the starting value to some place.
I.e. in database, or in function, or in site properties etc..
Once you have that, you can custom logic to generate your particular next serial number.
Now you need to again save that number so that you can get it next time when you will again increment it with one and use it. And this cycle continues each time you create new serial number.
Please check attached OML file which is generating exactly same thing which you required.
It will generate something like this.
Regards,
Palak Patel