import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        double pockyLength = sc.nextInt();
        double perCrunchLength = sc.nextInt();

        int numCrunch = (int) (Math.ceil(pockyLength / 2 * perCrunchLength) - 1);
        int eatPockyLenght = (int) (numCrunch * perCrunchLength);

        System.out.println(eatPockyLenght);

    }

}