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 Thu Apr 16 20:04:59 2020

@author: Equipo
"""

while True:
    n = int(input())
    if (n>=1 and n<=10**15):
        break
    
if (n%2==0):
    print(int(n/2))
else:
    print(-int((n+1)/2))