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
import sys
n = int(sys.stdin.readline())
a = n / 2
b = n / 2
c = n - 1
if n % 2 == 1:
    b = b + 1
    c = n

ans = (2 + a * 2) * a / 2 - (1 + c) * b / 2
sys.stdout.write(str(ans) + '\n');
