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())
even_start=2
odd_start=1
if(n%2==0):
    even_end=n
    odd_end=n-1
else:
    even_end=n-1
    odd_end=n
sum_of_even=(((even_end-even_start)//2 + 1)*(even_start+even_end))//2
sum_of_odd=(((odd_end-odd_start)//2 + 1)*(odd_start+odd_end))//2
print(sum_of_even-sum_of_odd)
