import java.util.Scanner;

public class No84 {

	public static void main(String[] args) {
		
		Scanner scanner = new Scanner(System.in);
		long r = scanner.nextLong() , c = scanner.nextLong();
		scanner.close();
		
		long ans;
		
		if(r != c) {
			if(r % 2 == 0) {
				ans = c * (r / 2) - 1;
			}else {
				ans = c * (r / 2) + c / 2 + c % 2 - 1;
			}
		}else {
			if(r % 2 == 0) {
				ans = r * r / 4 - 1;
			}else {
				ans = (r+1) * (r/2) / 2;
			}
		}
		
		System.out.println(ans);

	}

}