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
til = int(input())
if til % 2 == 0:
    n = int(til/2)
    odd = n*n
    even = n*(n+1)
    print(even - odd)
else:
    n = (til // 2) + 1
    odd = n*n
    even = (n-1)*(n)
    print(even - odd)
