import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
        int n = koko.nextInt();
        if(n%2==0){
            int a = n/2;
            System.out.println((long)n*n/4+n);
        }else{
            int a = (n+1)/2;
            System.out.println((long)-a*a+n*a+n);
        }
    }
    
}