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 -*-
additions = []
a = 0
n = int(input())
if n > 100 and n % 2 == 0: 
    print(int(n / 2))
if n > 100 and n % 2 != 0:
    print(-(int(n/2)+1))
if n < 100 or  n == 100:
    for i in range(1, n + 1):
        if i % 2 == 0:
            additions.append(i)
            a = sum(additions)
        else:
            additions.append(-i)
            a = sum(additions)
        if i == n:
            break
    print(a)