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
n = int(input())
#n, k, m = map(int, input().split())
#s = input()
#c = list(map(int, input().split()))
#a = [0] * 3
'''for i in range(n):
    c = list(map(int, input().split()))'''
if n % 2 == 1:
    l = n // 2 - n
else:
    l = n // 2
print(l)