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_b = n // 2

n_a = n - n_b
a1 = -1
b1 = 2
d = 2
an = a1 - d*(n_a - 1)
bn = b1 + d*(n_b - 1)

sa = ((a1 + an) * n_a)// 2
sb = ((b1 + bn) * n_b)// 2
print(sa + sb)