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
#CodeForces
#Calculating Function
#Python 3.7.2

import sys

n = int(input())
if(n % 2 == 0):
    n = n // 2
    odd = n*n
    even = n*(n+1)
    print( -odd + even) 
else:
    n = (n//2) + 1
    odd = n*n
    even = n*(n+1) - 2*n
    print( -odd + even) 

