import java.util.Scanner;
public class Aba{
	public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		long n,res = 0;
		n = sc.nextLong();
		while(n > 1){
			res += n-2;
			n -= 2;
		}
		System.out.println(res);
	}
}