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
number=int(input())
if number%2==0:
    n=int(number/2)
    even_number=int(n*(n+1))
    odd_number=int(n**2)
    print(even_number-odd_number)
else:
    n = (number // 2)+1
    even_number = int(n * (n + 1))
    odd_number = int(n ** 2)
    last_odd=number+1
    print(even_number-odd_number-last_odd)