import java.util.*;

public class Yukicoder_84{
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		long R = stdIn.nextLong();
		long C = stdIn.nextLong();
		long ans = 0;
		if(R==1 && C==1){
			System.out.println(0);
		}
		else{
			if(R%2==0){
				if(C%2==0){
					ans = R*C/2-1;
				}
				else{
					ans = (C-1)*R/2+R/2-1;
				}
			}
			else{
				if(C%2==0){
					ans = (R-1)*C/2+C/2-1;
				}
				else{
					ans = (R-1)*(C-1)/2-1;
				}
			}
			System.out.println(ans);
		}
	}
}