#!/bin/python

import sys
def sumOfGroup(k):
    begin = k*(k-1)//2
    sum = 0
    for i in range(begin,begin + k):
        sum+=i
    return (sum*2 + k)
    

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