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
if __name__ == '__main__':
    n = int(input().strip())
    pos = n // 2 if n % 2 == 0 else (n - 1) // 2
    neg = n if n % 2 == 1 else n - 1
    pos = (pos * (pos + 1))
    neg = ((neg + 1) // 2) ** 2
    print(pos - neg)
