import java.util.*;
public class Soroban{
	public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		long r,c,out;
		r = sc.nextInt();
		c = sc.nextInt();
		if(r == c){
			if(r %2 == 0){
				out = r*c /4 -1;
			}else{
				out = (r/2)*((r/2)+1);
			}
		}else{
			if(r%2 == 0 || c %2 ==0){
				out = r*c /2 -1;
			}else{
				out = r*c /2;
			}
		}
		System.out.println(out);
	}
}