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
num = int(input())

even = odd = 0

if num&1:
    odd = num
else:
    even = num

odd = odd if odd else num - 1
even = even if even else num - 1
n1 = (odd - 1) // 2 + 1
n2 = (even - 2) // 2 + 1

sum1 = -n1 * (1 + odd) // 2
sum2 = n2 * (2 + even) // 2

print(sum1 + sum2)