// editor1
import java.util.*;

class Main {
    int sum,i=0;
    int sumOfEven1(int n) {
        if(n==0) return sum;
        if(n%2==0) {
            sum+=n;
            n--;
            return sumOfEven2(n);
        }
    }
    
    int sumOfEven2(int n) {
        if(n%2==1) {
            n--;
            return sumOfEven1(n);
        }
        return sum;
    }
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        if(n<1) {
            System.out.println("Invalid input");
            return;
        }
        System.out.println(new Main().sumOfEven1(n));
    }
}

/*class Main {
    int sum,i=0;
    public int sumOfEven(int n) {
        i++;
        if(i%2 == 0) sum+=i;
        if(i==n) return sum;
        
        
        return sumOfEven(n);
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        if(n<1) {
            System.out.println("Invalid input");
            return;
        }
        System.out.println(new Main().sumOfEven(n));
    }
}*/