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
n = int(input())
if n % 2 == 0:
    odd = n // 2
    even = odd
else:
    odd = ((n - 1) // 2) + 1
    even = (n - 1) // 2
n = odd
S_odd = (n * (2 + (n-1) * 2)) // 2
n = even
S_even = (n * (4 + (n-1) * 2)) // 2
res = -1 * (S_odd) + S_even
print(int(res))
