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.Scanner;
import java.math.*;
public class t1{
	public static void main(String[] args) {
	// -1 2 -3 4 -5 6 -7 8 -9 10	
		Scanner scan=new Scanner(System.in);
		//long n=scan.nextInt();
		BigInteger n=scan.nextBigInteger();
		if(n.mod(BigInteger.valueOf(2)).equals(BigInteger.ZERO) ){
			//System.out.println("even");
			BigInteger res=n.divide(new BigInteger("2"));
			System.out.println(res);
		}else{
			//System.out.println("odd");
			BigInteger res=n.divide(new BigInteger("2")).subtract(n);
			System.out.println(res);
		}
		/*
		while(i<=n){
			sum=sum+(i)*(int)Math.pow(-1,i);
			i++;
		} */
		
		
		
	
	}
	
    
    
 }

