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
# -*- coding: utf-8 -*-
"""
Created on Sun Sep 10 18:28:52 2017

@author: svpopo18
"""


def F(n):
    if n % 2 != 0:
        f = -(n + 1) // 2
    else:
        f = (n + 1) // 2
    return f

n = int(input())

print(F(n))
