#!/bin/python

import sys

def sumOfGroup(k):
    # Return the sum of the elements of the k'th group.
    i=1
    n=1
    while i<k:
        i += 1
        n += i
    return (n**2 - (n-i)**2)

if __name__ == "__main__":
    k = int(raw_input().strip())
    answer = sumOfGroup(k)
    print answer