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())
if(n%2 ==0):
    even_n = n
    odd_n = n-1
else:
    even_n = n-1
    odd_n = n

last_even = int(even_n/2)
last_odd = int((odd_n+1)/2)

sum_even = last_even * (last_even + 1)
sum_odd = last_odd ** 2

print(sum_even - sum_odd)