I want to update a count on Contact object after a new record is inserted or deleted on Lents object. My code is working fine to update the count by 1 but is not decreasing its value by 1 please help me:

trigger UpdateCount on Lent__c (after insert,after delete) {
List<Contact> counts_toUpdate = new List<Contact>();
Map<String, Integer> contact_newCount_Map=new Map<String, Integer>(); //for each contact the CountNumber

if(trigger.isinsert){
List<Lent__c> counts =[select Contact__c, Contact__r.Count__c
from Lent__c where id IN :Trigger.new FOR UPDATE];
for(Lent__c lent:counts){
contact_newCount_Map.put(lent.Contact__c, Integer.valueOf(lent.Contact__r.Count__c==null ? 0 : lent.Contact__r.Count__c) + 1);
}
}
else{
Set<Id> contactIds = new Set<Id>();
for (Lent__c l : Trigger.old)
contactIds.add(l.Contact__c);
List<Lent__c> counts = [select Contact__c, Contact__r.Count__c
from Lent__c where id IN :contactIds];
Map<Id,Contact> oldContacts = new Map<Id,Contact>([SELECT Count__c From Contact where Id IN :contactIds]);
for(Lent__c lent : counts)
contact_newCount_map.put(lent.Contact__c, Integer.valueOf(oldContacts.get(lent.Contact__c).Count__c == null ? 0 : oldContacts.get(lent.Contact__c).Count__c ) -1);
}

for(Contact con : [select id, Count__c from Contact WHERE ID IN : contact_newCount_Map.KeySet()]){
con.Count__c=contact_newCount_Map.get(con.id);
counts_toUpdate.add(con);
}

update counts_toUpdate;
}