import java.util.Scanner;

public class Main84 {
	public static void main(String[] args) {
		Main84 p = new Main84();
	}

	public Main84() {
		Scanner sc = new Scanner(System.in);
		long R = sc.nextLong();
		long C = sc.nextLong();
		solve(R, C);
	}

	public void solve(long r, long c) {
		
		long res = -1;
		if(r==1 || c==1)
			res += (r*c+1)/2;
		else if(r==c)
			res += (r*c-r-c+1)/4+(r+1)/2 + r%2;
		else
			res += r*(c/2) + (r+1)/2 * (c%2);
		
		System.out.println(res);
	}

}