import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
int n = scan.nextInt();
int sum = 0;
for(int i = 1; i <= n; i++) {
if(i % 2 == 0) {
sum = sum - i;
} else {
sum += i;
}
}
System.out.println(sum);
}
}