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/2 + (r%2==0 ? 0 : 1);
		}else{
			res += r*(c/2);
			if(c%2==1)
				res += (r+1)/2;
		}
		
		
		System.out.println(res);
	}

}