Create a number sequence in a custom field on the lead

How to generate a number sequence every time the user creates a new lead?

In Opportunities, create a trigger to have the following sequence: last two digits of the current year + Numerical sequence

def currYear = year(today());

def vo = newView('OpportunityVO');

def vc = newViewCriteria(vo);

def vcr = vc.createRow(); NDeProyecto_c

def vci1 = vcr.ensureCriteriaItem('NDeProyecto_c');

vci1.setOperator('>');

vci1.setValue(0);

def vci2 = vcr.ensureCriteriaItem('OptyCreationDate');

vci2.setOperator('AFTER');

vci2.setValue(dateTime(currYear,01,01,01,01));

vc.insertRow(vcr);

vo.setSortBy('OptyCreationDate desc');

vc.insertRow(vcr);

vo.appendViewCriteria(vc);

vo.executeQuery();

def linha = vo.first();

println(linha);

if(linha != null){

 def id = linha.NDeProyecto_c;