import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		long R = scan.nextLong();
		long C = scan.nextLong();
		scan.close();
		if(R == 1 && C == 1) {
			System.out.println(0);
			System.exit(0);
		}
		if(R % 2 == 1 && C % 2 ==1) {
			System.out.println(R * (C / 2) + R / 2);
		}else {
			if(R % 2 == 0) {
				System.out.println((R / 2) * C - 1);
			}else{
				System.out.println(R  * (C / 2) - 1);
			}
		}
	}
}