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 Main {
		public static void main(String[] args) throws Exception{
		BufferedReader b = new BufferedReader(new InputStreamReader(System.in));
		BigInteger n = new BigInteger(b.readLine());
		BigInteger two = new BigInteger(String.valueOf(2));
		if (n.mod(two).equals(BigInteger.ZERO))
			System.out.println(n.divide(two));
		else
			System.out.println("-"+(n.add(BigInteger.ONE)).divide(two));
		b.close();
	}
}

 			  	 	 		 	 			 				  	 			