salesforce - How can I get a count of number of records in child object on parent object field? -

i have object participant child of interaction. on editing child object records field in parent object should incremented. scenario such first time if invited true , rsvp null invited count on parent object should incremented. next same record if rsvp set yes rsvp field on parent object should incremented. logic getting incremented in both invited rsvp. below trigger

set<id> interactionids = new set<id>(); (participant__c erpaccount : participantlist){     interactionids.add(erpaccount.interaction__c); }  list<interaction__c> relatedinteractions =     [select i.total_number_of_invited_attendees__c,         (select invited__c,rsvp_status__c,hcp__c,interaction__c   participants__r             role_group__c = 'attendee'                         , (invited__c = true                         or rsvp_status__c = 'yes'                         or hcp__c !=null))         interaction__c id in : interactionids];  for(interaction__c acc : relatedinteractions){     //acc.total_number_of_invited_attendees__c = 0;       for(participant__c p : acc.participants__r){         if(p.invited__c == true)         {                   if(p.rsvp_status__c == 'yes')             {                 if(p.hcp__c != null)                 {                    acc.total_no_of_confirmed_customer_attendees__c += 1;                 }                 else                 {                     acc.total_number_of_confirmed_attendees__c += 1;                                           }             }             else             {                 acc.total_number_of_invited_attendees__c += 1;             }         }            } }  update relatedinteractions; 

thanks in advance !!