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; System.out.println(R * C / 2L - b); } }