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.InputStreamReader;
import java.math.BigInteger;

public final class CalculatingFunction {
	public static void main(String args[]) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		BigInteger N = new BigInteger(br.readLine());
		BigInteger two = new BigInteger("2");
		BigInteger Result = N.divide(two);
		if (N.mod(two).equals(BigInteger.ZERO)) {
			System.out.println(Result);
		} else {
			System.out.println("-" + (Result.add(BigInteger.ONE)));
		}
	}
}
