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
def sum_to_n(num):
    val = (num * (num + 1) ) / 2
    return val

num = int(raw_input())

even = 0
odd = 0
if num % 2 == 0:
    even = num
    odd = num / 2
else:
    even = num - 1
    odd = num / 2
    odd += 1

even_sum = 2 * sum_to_n(even / 2)
odd_sum = 2 * sum_to_n(odd - 1) + odd
val = sum_to_n(odd) - even_sum

print even_sum - odd_sum


