import java.util.Scanner;
public class Main{
    public static int ntosum(int n){
        if (n==0){
            return 0;
        }else if(n%2==0){
            int sum = 0;
            sum+=n;
            return n + ntosum(n-2);
        }else{
        return ntosum(n-1);
    }
    }
    
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        if (n>0){
            System.out.println(res);
        }else{
            System.out.println("Invalid input");
        }
        int res=ntosum(n);
        
        
        
    }
}