// editor4
import java.util.Scanner;
public class main{
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        double b = input.nextFloat();
        double h = input.nextFloat();
        double a = b*h/2;
        if(a<0 || b<0 || h<0) {
            System.out.print("Invalid input");
        }
        System.out.printf(a);
    }
}