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.

The single line contains the positive integer n (1 \le n \le 1015).

Print f(n) in a single line.

f(4)=-1+2-3+4=2

f(5)=-1+2-3+4-5=-3