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())

r2 = 0

if n == 1:
	print( -1)
elif n == 2:
	print(1)
elif n == 3:
	print(-2)
else:



	if n%2==0:
		r2 += (n//2)*(2 + (n // 2 - 1))
		r2-=(n//2)*(1+(n//2-1))
	else:
		r2 += (n//2)*(2+(n // 2 - 1))
		r2-=((n//2+1))*(1+(n//2))













	print(r2)      