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 calculating_func(n):
	import math
	ceil =  math.ceil(n/2) if n%2!=0 else n//2
	odd =  ceil**2
	even = (n//2)*((n//2)+1)
	print(even-odd)


if __name__ == '__main__':
	calculating_func(int(input().strip(" ")))	
