import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long h = sc.nextInt();
        long w = sc.nextInt();
        long ans;
        if (h == w) {
            if (h % 2 == 0) {
                ans = h * w / 4 - 1;
            } else {
                ans = (h * w - 1) / 4;
            }
        } else {
            if (h % 2 == 0 || w % 2 == 0) {
                ans = h * w / 2 - 1;
            } else {
                ans = (h * w - 1) / 2;
            }
        }
        System.out.println(ans);
    }
}