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;
import java.util.*;

public class Main {
	
	public static void main(String []args) throws IOException {
		Scanner sc = new Scanner(System.in);
		BigInteger n = sc.nextBigInteger();

		BigInteger bTWO = BigInteger.ONE.add(BigInteger.ONE);
		BigInteger bONE = BigInteger.ONE;
		BigInteger sumPar = BigInteger.ZERO;
		BigInteger sumImpar = BigInteger.ZERO;

		if (n.mod(bTWO).compareTo(BigInteger.ZERO) == 0){
			sumPar = (bTWO.add(n)).multiply((n.divide(bTWO))).divide(bTWO);
			sumImpar = n.multiply((n.divide(bTWO))).divide(bTWO);
		} else {
			sumPar = (bONE.add(n)).multiply((n.divide(bTWO))).divide(bTWO);
			sumImpar = (bONE.add(n)).multiply((n.divide(bTWO).add(bONE))).divide(bTWO);
		}

		System.out.println(sumPar.subtract(sumImpar));
	}
} 
// 1474657929068