import java.util.*;
import java.math.*;

public class Main {
    private static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) throws Exception {
        long r = sc.nextLong();
        long c = sc.nextLong();
        long ret;
        boolean b = r%2==0 || c%2==0;
        if (r == c) {
            if (b) {
                ret = r*c/4-1;
            } else {
                ret = (r*c-1)/4;
            }
        } else {
            if (b) {
                ret = r*c/2-1;
            } else {
                ret = (r*c-1)/2;
            }
        }
        
        System.out.println(ret);
    }
}