import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
       long r = koko.nextLong();
       long k = koko.nextLong();
       long a;
       if(k==r&&k%2==0){
           a = r*k/4;
       }else if(k==r&&k%2!=0){
           a = 1+(r*k-1)/4;
       }else if(k!=r&&k%2!=0&&r%2!=0){
           a = 1+(r*k-1)/2;
       }else{
           a = r*k/2;
       }
       System.out.println(a-1);
    }
}