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())

if n % 2 == 0:
    a = n // 2 * (n // 2 + 1)
    b = n * n // 4
    print(a - b)
    # print(((n // 2) * ((n // 2) + 1)) - ((n ** 2) // 4))
else:
    # print((((n - 1) // 2) * ((n - 1) // 2) + 1) - ((n + 1) // 2) ** 2)
    t = (n - 1) // 2
    c = t * (t + 1)
    d = ((n + 1) // 2) ** 2
    print(c - d)