How to write a Salesforce trigger for below given scenario?

trigger concatinat on Contact (after insert,after update)
{
List accList = new List();
set S = new Set();
for(contact con : Trigger.new)
{
S.add(con.accountid);
}
Map mv = new Map([SELECT id,name
FROM account WHERE Id IN : S]);

for(contact cont : Trigger.new)
{
Account acct = new Account();
acct.id = mv.get(cont.accountid).id;
acct.name = mv.get(cont.accountid).name + '-' +cont.lastname;
accList.add(acct);

}
update accList;
}