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())
m  = n-1
if n%2==0:
    a = ((n//2)*(m+2))//2
    b = ((n//2)*(1+m))//2
    s = (n//2)*(n+2)//2-((n//2)*(1+m))//2
    print(s)
else:
    s = (((n-1)//2)*(2+m))//2-(((n+1))//2*(1+n))//2
    print(s)