import java.util.Arrays; import java.util.LinkedList; import java.util.Scanner; public class Main { // 期待値の線形性 E[∑_i Xi] = ∑_i E[Xi] より, ∀i. E[Xi] = 3.5 より, 3.5 * N public static void main(String[] args){ Scanner sc = new Scanner(System.in); final long R = sc.nextLong(); final long C = sc.nextLong(); System.out.println(Math.max(0, (R * C / (R == C ? 4 : 2)) - (R * C % 2 == 0 ? 1 : 0))); } }