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);

}

}