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():
    n = int(input())

    if n == 1:
        print(-1)
        return
    if n == 2:
        print(1)
        return

    if n % 2:
        sum_odds = (n // 2 + 1) ** 2
    else:
        sum_odds = (n // 2) ** 2

    print(2 * sum_cons(n // 2) - sum_odds)

def sum_cons(n):
    return n * (n + 1) // 2

if __name__ == "__main__":
    main()