import java.util.*;
public class Main {
    public static void main(String[] args)
    {
        Scanner cs = new Scanner(System.in);
        int N = cs.nextInt();
        if (N < 1) {
            System.out.println("Invalid input");
            return;
        } 
        sum = 0 ;
        for(int i = 1 ; i < N ; i += 2 )
            {
                sum += i;
            }
        System.out.println(sum);
        }
    }