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())
# fn = [((-1)**n)*n for n in range(n+1)]
# print(fn)
# a = -1 
# l= (-1)**n
# s= sum((-1)**(k) * k for k in range(1, n + 1))
n_l = n%2
if n_l == 0:
	n_ = n//2
else:
	n_ = int(n//2 + ((-1)**n)*n)
print(n_)