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
n = int(input())
len1 = n // 2
len2 = n - len1
ls1 = 0
ls2 = 0
if n % 2 == 0:
	ls1 = n
	ls2 = n - 1
else:
	ls1 = n - 1
	ls2 = n
s1 = (2 + ls1) * len1 // 2
s2 = (1 + ls2) * len2 // 2
print(s1 - s2)