[WmyEE0UsWAwC2i]

I don't think that's the best way to use Optional in java. In particular the use of isPresent & get kind of defeats the purpose. Should be something along the lines :

   ...
   sum = 0
   opt = getOptional(n)
   sum = sum + opt.orElse(0);
   ...

Also maybe use something like OptionalInt (but for long).