import java.util.*; import java.io.*; import java.math.*; public class No84 { public static void main(String[] args) { Scanner sc = new Scanner(System.in); long r = sc.nextLong(); long c = sc.nextLong(); long a = 0; if(r == c) a = (long)(r * c / 4); else if(r * c % 2 == 0) a = (long)(r * c / 2 - 1); else a = (long)(r * c / 2); if(a < 0) a = 0; System.out.println(a); } }