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
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Sun Nov  1 01:38:21 2020

@author: Error
"""

n = int(input())

summe = 0

if n % 2 == 0:
    summe = n/2
else:
    zwischen_summe = (n-1)/2
    summe = zwischen_summe-n
print(int(summe))
    
    