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.IOException;
import java.io.InputStreamReader;

public class cf486a {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        long n = Long.parseLong(br.readLine());
        System.out.println(myFunction(n));
    }

    private static long myFunction(long n) {
        if(n==1)
            return -1;
        long sum = 0;

        long numOfTerms = ((n-2)/2)+1;
        sum+= numOfTerms * ((n+2)/2);

        numOfTerms = ((n-1)/2)+1;
        sum-= numOfTerms * ((n+1)/2);

        return sum;
    }


}
