package no072;

import java.math.BigInteger;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger n = new BigInteger(sc.next());
		BigInteger half = n.divide(new BigInteger("2"));
		System.out.println(
				half.add(BigInteger.ONE).multiply(n.subtract(half).add(BigInteger.ONE)).subtract(BigInteger.ONE));
	}

}