Competitions

# Card houses

Slavik wants to become an artist of original genre, so he constantly engaged in manipulation with a variety of objects. At the moment, all his spare time he is busy in building houses of cards. He has even managed to build a 4 - storey house, where he used 26 cards. At the technology lesson secretly from the teacher he demonstrated his ability before classmates to build a house with 3 floors, with just 15 cards. "Arts" of the future artist are given in the pictures from his mobile phone below.

Currently Slavik is interested in the question of mathematical content: how many cards you need to have to build a house with n floors? He asks you to help him, as with mathematics Slavik has problems ...

#### Input

Positive integer n - the number of the floors in a house to be built. This number is no more than 108.

#### Output

Print the number of cards needed for the construction of a house with n floors.

Time limit 1 second
Memory limit 128 MiB
Input example #1
3

Output example #1
15

Input example #2
4

Output example #2
26

Author Анатолий Присяжнюк
Source 2013-2014 2-nd stage All Ukrainian Informatics Olympiad, class 9, Berdichev