I have got error :Error: Compiled formula is too big to execute (6,715 characters). Maximum size is 5,000 character.

My requirement is:

Calculate scores based on selection  checkbox:

Visualforce page:

     question               P - 0                       p--1                 p--2                 p--3             p--4

A                                  chechkbox           chechkbox

B 

C

D

E

F

G

H

I

J

K 

L                           chechkbox               chechkbox                                                   chechkbox

:(Sample for first question formula field name is A)

A=.IF(P0__c = true , 0 , 
   IF(p1__c = true , 1 , 
  IF(p2__c= true , 2 , 
   IF(p3__c= true , 3 , 
    IF(p4__c = true ,4 , null )))))

I have created  formula field  for  12 Questions (A to L) LIKE ABOVE

Then created  total formula field ---Total =A__c+B__c+C__c+D__c+E__c+F__c+G__c+H__c+I__c+J__c+K__c+L__c..

This is the my requirement . But i have got Error: Compiled formula is too big to execute (6,715 characters). Maximum size is 5,000 characters.How to reduce Compiled size in Formula field ..OR How to use this logic in apex code .Please give ur idea For this task.I dont want to work flow field update process. because i want to update existing record also..