import java.util.Scanner;

public class No84 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long H = sc.nextLong();
		long W = sc.nextLong();
		
		if(H == W) {
			if(H % 2 == 1) {
				System.out.println(H * W / 4);
			}else {
				System.out.println(H * W / 4 - 1);
			}
		}else {
			if(H * W % 2 == 1) {
				System.out.println(H * W / 2);
			}else {
				System.out.println(H * W / 2 - 1);
			}
		}
	}
}