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
def f(n) :
	if n == 0 :
		return 0
	elif n == 1 :
		return -1
	else :
		if n % 2 == 0 :
			return n//2
		elif n %  2 == 1 :
			return f(1) * (f(n-1) + 1)
if __name__ == '__main__':
	x = int(input())
	print(f(x))

