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
numero = int(input())
cociente = numero//2
pares = 2*((cociente*(cociente+1))/2);
if(numero%2==0):
    impares = (((numero-1)*numero)/2)-2*((cociente*(cociente-1))/2);
else:
    impares = (((numero+1)*numero)/2)-pares;

print(int(pares-impares))
