#!/bin/python

import sys
def sumOdd(n):
    return 2*((n*(n+1))/2)-n
def sumOfGroup(k):
    g=k
    k-=1
    first=(k*(k+1))/2
    lower=first
    upper=lower+g
    return sumOdd(upper)-sumOdd(lower)
                
    # Return the sum of the elements of the k'th group.

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