For a positive integer n let's define a function f : f(n) = - 1 + 2 - 3 + .. + ( - 1) n n Your task is to calculate f(n) for a given integer n .

输入描述:

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

输出描述:

Print f(n) in a single line.

备注:

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