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 Wed Apr 17 20:22:19 2019

@author: KIIT
"""

import math
import os
import random
import re
import sys
from itertools import permutations

if __name__ == '__main__':
    n=int(input())
    even,odd,s=0,0,0
    if(n%2==0):
        even=odd=n//2
        s1=(even)*(2+n)//2
        s2=odd*(-1-(n-1))//2
        s=s1+s2
    else:
        even=n//2
        odd=(n+1)//2
        s1=even*(2+(n-1))//2
        s2=odd*(-1-n)//2
        s=s1+s2
print(s)
    