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