http://acmp.ru/index.asp?main=task&id_task=2

task02.py

import sys


def Sum(N):
	S = 0
	k = 1
	while  k <= N:
		S = S + k
		k = k + 1
	return S

def Sum2(N):
	return N*(N+1)//2

N, = [int(s) for s in sys.stdin.readline().strip().split()]

if N < 1:
	r = -Sum(-N) + 1
	print (r)
else:
	r = Sum(N)
	print (r)