I am trying to map total formula field in class but it will through error like this :Error: NewOrderPageClass Compile Error: Field is not writeable: Order_Product__c.Total__c at line 54 column 14
  can you please suggest me how to resolve this issue.

public with sharing class NewOrderPageClass
{
  public Id orid{get;set;}
  // public List<id> Opids{get;set;}
  Public Map<id,Account> mapAccounts{get;set;}
  public Order__c OppId{get;set;}
  public List<Order_Product__c> oplist{get;set;}
  public List<Product_Master__c> Pmid{get;set;}
  public NewOrderPageClass(ApexPages.StandardController controller)
      {
      try
          {
       String orid=ApexPages.CurrentPage().getParameters().get('id');
       mapAccounts=new Map<id,Account>([Select id,Name,RollupOrders__c from Account]);
       oplist=new List<Order_Product__c>();
       OppId=[select id,name,Account__c,Total_Quantity_Sets__c,Total_Order_Amount__c from Order__c where id=:orid];
            Pmid=[select id,name,Active__c,Size_S__c,Size_L__c,Size_M__c,Total__c,Size_XL__c,Size_XXL__c,Price__c,Product_Color__c,Remarks__c   from Product_Master__c WHERE
                    Product_Type__c='Sales' AND Active__c=True AND Thermax__c=False ORDER BY CreatedDate ASC];
       System.Debug('----->>>>>@@@@Size'+Pmid.size());
       System.Debug('----->>>>>@@@@Prose'+Pmid);
          }
        
      catch(Exception e)
          {
          system.debug(e);
          ApexPages.addMessages(e);
          }
      }
    public PageReference CreateOrderProduct()
        {
       try
         {
       for( Product_Master__c p:pmid)
         {
        
         if(p.Size_S__c>0||p.Size_L__c>0||p.Size_M__c>0||p.Size_XL__c>0||p.Size_XXL__c>0)
         {
          RecordType rty = [SELECT Name, id FROM RecordType WHERE sObjectType='Order_Product__c' and name='Sales'];
          System.debug('-------------------> Record Type Ids'+rty);
           Order_Product__c op=new Order_Product__c(RecordTypeId=rty.id);             
             op.Product_Master__c=p.id;
             op.Sales_Order__c=OppId.id;
            // op.name=p.Name;      
             op.Size_S__c= p.Size_S__c;
             op.Size_M__c= p.Size_M__c;
             op.Size_L__c= p.Size_L__c;
             op.Size_XL__c= p.Size_XL__c;
             op.Size_XXL__c= p.Size_XXL__c;
             op.Total__c = p.Total__c;
             op.Remarks__c = p.Remarks__c;             
             /*op.quantity__c=p.Quantity__c;*/
            
             System.debug('-------------------> Order Products are Ids'+op);
             oplist.add(op);                               
        }
        }
        }        
        catch(Exception e)
            {
            system.debug(e);
            }
        if(oplist.Size()>0)
        {
            insert oplist;
        }
      PageReference reference=new PageReference('/'+Apexpages.CurrentPage().getParameters().get('Id'));
        
        return reference;
       }
      
}