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 math
def BuubleSort(n):
	if n==1:
		return -1
	if n==2:
		return 1
	if n==3:
		return 2
	if n==4:
		return 2
	no = math.ceil(n/2)
	ne = n - no
	so = no**2
	se = ne*(ne+1)
	return se - so

	

def main():
	n = int(input())
	#l = list(map(int  , input().split()))
	a = BuubleSort(n)
	print(a)


if __name__ == "__main__":
	main()