Problem #44: House of Cards

Do you know how to build card houses?

The first floor is easy: /\ (2 cards)

The second floor is easy, too: 

 /\
 --
/\/\
(7 cards)

Here's the third: 

  /\
  --
 /\/\
 ----
/\/\/\
(15 cards)

And so forth.

How many cards does it take to build a 47-story house?

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