import java.util.Scanner;

public class Yukicoder84 {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long R = sc.nextLong(), C = sc.nextLong();
		long b = R % 2 == 0 || C % 2 == 0 ? 1 : 0;
		if (R == C) {
			R /=2L ;
			C += 1 - b;
		}
		System.out.println(R * C / 2L - b);
	}
}