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