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 Sat Nov  3 19:41:17 2018

@author: pc
"""

def f(x):
    if x%2==0:
        return x/2
    else:
        return f(x-1)-x
print(int(f(int(input()))))