Given a non-negative integer \(n\), compute the alternating sum

\(S(n) = -1 + 2 - 3 + 4 - \cdots + (-1)^n n\).

For \(n = 0\), define \(S(0) = 0\).

Input

One line containing \(n\).

The input satisfies:

• \(0 \le n \le 10^9\)

Output

Print \(S(n)\).

Example

Input

5

Output

-3