import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner inScanner = new Scanner(System.in);
        int m = 0, n = 0;
        int count = 0;
        m = inScanner.nextInt();
        n = inScanner.nextInt();
        if (m * n < 2)
            count=0;
        else if (n == 1) {
            count += (m / 2);// downward
        } else {

            int npart = n / 2;
            count += (npart * m);
            if(n%2==1){
                count+=(m/2);// downward
            }
        }
        System.out.println(count);
    }

}