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 f(n):
    if n%2 == 0:
        k1 = int(n/2)
        return k1*(k1+1) - (k1*k1)
    else:
        k2 = int((n+1)/2)
        return -(k2*k2) + (k2*(k2-1))
n = int(input())
print(f(n))