#!/bin/python3

import sys
#a=[-1]*10000000
def reck(n):
	#print('abc')
    return n+(((n-2)*(n-1))//2)
    
def sumOfGroup(k):
    # Return the sum of the elements of the k'th group.
    start=(reck(k));
    #print(start)
    su=0
    for i in range(start,start+k):
           su+=i
    return ((2*su)-k )

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