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
def main():
    # easy to see that the score is going like
    # ---->  -1 / 1 / -2 / 2 / -3 / 3 / ...
    n = int(input())
    if 1 <= n <= pow(10,15):
        if n%2 == 0:
            print(int(n/2))
        else:
            print(int(((n+1)/2) * (-1)))
main()