The $$Funny$$ $$Book$$ has its pages numbered in the following way: $$1, 2, 3, 3, 3, 4, 4, 4, 4, 5, \cdots$$. That is there is one $$1$$, two $$2s$$, three $$3s$$, four $$4s$$, five $$5s$$, and so on. How many actual pages (including front and back) will the $$Funny$$ $$Book$$ have if it contains all possible pages that are numbered "$$1$$" through "$$20$$", but not any that are numbered "$$21$$"? 
Overseas Competition->Knowledge Point->Calculation Modules->Sequences and Number Tables->Arithmetic Sequences