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 = raw_input()
    n = int(n)

    eve_ap_a = 2
    odd_ap_a = -1

    eve_ap_n = int(n/2)
    odd_ap_n = int(n/2) if n % 2 == 0 else int(n/2) + 1

    eve_ap_an = n if n % 2 == 0 else n - 1
    odd_ap_an = -1 * n if n % 2 != 0 else -1 * (n - 1)

    eve_ap_sum = (eve_ap_n * (eve_ap_a + eve_ap_an)) / 2
    odd_ap_sum = (odd_ap_n * (odd_ap_a + odd_ap_an)) / 2

    print (eve_ap_sum + odd_ap_sum)
    return 

if __name__ == "__main__":
    main()