House of Cards

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

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.