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
from math import  ceil


def solve(n):
    if n == 1: return -1
    l_odd = n if n & 1 else n - 1
    x = -((1 + l_odd)*ceil(l_odd/2))//2
    l_even = n if n % 2 == 0 else n-1
    y = ((2 + l_even)*ceil(l_even/2))//2
    return x + y


def main():
    n = int(input())
    print(solve(n))


main()
