import java.util.Scanner;


public class Main {
	public static void main(String[] a) {
		Scanner s=new Scanner(System.in);
		long R=s.nextLong(),C=s.nextLong(),S=R*C%2>0?R==C?(R-1)*(R+1)/4:R*C/2:R*C/(R==C?4:2)-1;
		System.out.println(S);
	}
}