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≤n≤1015).
Print f(n) in a single line.
4
2
5
-3
f(4)=-1+2-3+4=2
f(5)=-1+2-3+4-5=-3