import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		final long R = sc.nextLong();
		final long C = sc.nextLong();
		
		System.out.println(Math.max(0, (R * C / (R == C ? 4 : 2)) - (R * C % 2 == 0 ? 1 : 0)));
	}
	
}