Description: 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 Format: The single line contains the positive integer n (1 ≤ n ≤ 1015). Output Format: Print f(n) in a single line. Note: f(4) = - 1 + 2 - 3 + 4 = 2 f(5) = - 1 + 2 - 3 + 4 - 5 = - 3