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.math.BigInteger;
import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.TreeSet;
public class Main  {
	public static void main(String[] args) {
	
		Scanner s=new Scanner(System.in);
		
		BigInteger x=s.nextBigInteger();
		
		BigInteger n=x.multiply(x.add(BigInteger.valueOf(1)));
		BigInteger n1=n.divide(BigInteger.valueOf(2));
		//System.out.println(n1);
		//System.out.println(n);
		
		BigInteger fe=BigInteger.valueOf(2);
		BigInteger le;
		if(x.mod(BigInteger.valueOf(2))==BigInteger.valueOf(0)){
			le=x;
		}else{
			le=x.subtract(BigInteger.valueOf(1));
		}
		//System.out.println(le);
		
			BigInteger o=fe.add(le);
			BigInteger o1=o.divide(BigInteger.valueOf(2));
			BigInteger o2=o1.subtract(BigInteger.valueOf(1));
			BigInteger p=o2.multiply(o2.add(BigInteger.valueOf(1)));
			BigInteger res=n1.subtract(p);
			
			BigInteger res1=p.subtract(res);
			System.out.println(res1);
			
			
		
			
		

		
	}
}

