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

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

Output

Print f(n) in a single line.

Examples

Input

4


Output

2


Input

5


Output

-3

Note

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

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3
import java.io.*;
import java.math.BigInteger;

public class Practice
{
    public static void main(String[] args) throws IOException
    {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        BigInteger n = new BigInteger(reader.readLine());
        BigInteger total = n.add(new BigInteger("1")).divide(new BigInteger("2"));
        
        if(n.mod(new BigInteger("2")).equals(new BigInteger("0"))) System.out.println(total);
        else System.out.println(total.negate());
    }
}