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
'''    #this code is right but time exceeded.
n = int(input())
if n>0 and n<=1000000000000000:

    i = 1
    sum = 0
    while i<=n:
        if i % 2 == 1:
            sum -= i
        else:
            sum += i
        i += 1
    print(sum)
    '''
from math import *
n = int(input())
if n>0 and n<=1000000000000000:
    print(ceil(n / 2) * ((-1) ** n))

