package codeup100;

import java.util.Scanner;

public class c1078 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num = sc.nextInt();
        sc.close();

        int sum = 0;
        for(int i=0; i<=num; i++){
            if(i%2 == 0){
                sum += i;
            }
        }
        System.out.println(sum);
    }
}