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
import sys
import math
input = sys.stdin.readline

def int_array():
    return list(map(int, input().strip().split()))


def str_array():
    return input().strip().split()

def lower_letters():
    lowercase = []
    for i in range(97, 97+26):
        lowercase.append(chr(i))
    return lowercase
    
def upper_letters():
    uppercase = []
    for i in range(65, 65+26):
        uppercase.append(chr(i))
    return uppercase

######################## TEMPLATE ENDS HERE ########################
n = int(input())
if n % 2 == 0:
    print(n//2)
else:
    print(-(n+1)//2)