For a positive integer n let's define a function f:

f(n) = - 1 + 2 - 3 + .. + ( - 1)nn

Your task is to calculate f(n) for a given integer n.

Input

The single line contains the positive integer n (1 ≤ n ≤ 1015).

Output

Print f(n) in a single line.

Examples

Input

4


Output

2


Input

5


Output

-3

Note

f(4) = - 1 + 2 - 3 + 4 = 2

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3
import re
def gcd(x, y):
	while(y):
		x, y = y, x % y
	return x
def lcm(x,y):
	return (x*y)/gcd(x,y)
lucky_nums = [4,7,44,47,74,77]
def gen(nums):
	even = int(nums/2)
	odd = nums - even
	even = even*(even+1)
	odd = odd**2
	return (even-odd)

def dothis():
	
	return map(lambda x: int(x),raw_input().split(" "))
def main():
	# nums = []
	nums = int(raw_input())
	# nums = raw_input().split(" ")
	# word1 = raw_input()
	# word2 = raw_input()
	# puzzles = raw_input().split(" ")
	#nums = map(lambda x: int(x),nums)
	#nums = [int(raw_input()) for t in range(0,3)]
	print gen(nums)

if __name__ == "__main__":
	# while True:
	# 	main()
	main()