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 Feb 12 18:34:46 2017

@author: mehdishakiba
"""

### The following code couldn't handle large numbers apparently... 

#n = int(raw_input())
#
#res = long(0)
#sign = 1
#
#for i in range(1,n+1):
#    sign = sign*(-1)
#    res += sign*i
#
#print res





# So I'm just solving the problem. Though there should be a fix to the above version that numerically finds the solution instead of solving it like this

n = int(raw_input())

if n%2 == 0:
    res = n/2
else:
    res = n/2 - n
    
print res