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
from sys import stdin

arr = [int(x) for x in stdin.readline().rstrip().split()]

num = arr[0]

def compEven(num): # precond num is even
	a1 = 2
	an = num
	n = num // 2
	Sn_even = n * (a1 + an) // 2

	a1 = 1
	an = num - 1
	n = num // 2
	Sn_odd = n * (a1 + an) // 2

	return Sn_even - Sn_odd

def comp(num):

	if num == 1:
		print('-1')
		return

	if num % 2 == 1:
		print(-1 * num + compEven(num - 1))
	else:
		print(compEven(num))

comp(num)