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(raw_input())

if n % 2 == 0:
    sum1 = (n/2 * (2+n)) / 2
    sum2 = (n/2 * n) / 2
    ans = sum1 - sum2
else:
    sum1 = ((n+1)/2 * (n+1)/2)
    sum2 = ((n-1)/2 * (n+1)/2)
    ans = sum2 - sum1

print ans