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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;


public class Codeforces486A
{
	public static void main(String[] args)
	{
		try
		{
			BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
			BigInteger n = new BigInteger(f.readLine());
			BigInteger two = new BigInteger("2");
			BigInteger answer = n.divide(two);
			if(n.mod(two).subtract(BigInteger.ONE) == BigInteger.ZERO)
				answer = answer.subtract(n);
			System.out.println(answer);
		}
		catch(IOException e)
		{
			System.out.println(e);
		}
	}
}
