Write a Salesforce Trigger for account name should be a combination of lastnames of all its related contacts

trigger contactlastName on Contact (after insert,after update)
{
list lst=new list();
for(contact c:Trigger.new)
{
lst.add(c.accountId);
}
list lac=new list();
list lacc=[select Id,Name,(select Id,lastName from contacts) from account where Id in:lst];
for(account a:lacc)
{
for(contact con: a.contacts)
{
a.Name=a.Name+' '+ con.lastName;
lac.add(a);
}
}
update lac;
}

this trigger works only one time,after run this trigger second related contact is not created,plz help

yes and thanks