House of Cards

Published by Matt in
mathnumbers

The image below shows a sequence of larger and larger houses of cards, with the total number of cards included for each:

House of Cards

Given the tower height n, return the number of cards needed to construct a full house of cards.

Examples

cards_needed(3) ➞ 15

cards_needed(4) ➞ 26

cards_needed(0) ➞ 0

Notes

  • The height should always be equal or greater than 0.
  • Return "invalid" if the value given doesn't abide by this rule.
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