import java.util.*; import static java.lang.Math.*; public class ppp { public static void main(String[] args) { Scanner sc=new Scanner(System.in); long r=sc.nextInt(); long c=sc.nextInt(); long x=0; if(r==c){ x=(r*r/4); if(r%2==1){ x+=r/2+r/2+1; } } if(r!=c){ x=(r/2)*(c/2)*2; if(r%2==0 && c%2==1){ x+=r/2; } if(r%2==1 && c%2==0){ x+=c/2; } if(r%2==1 && c%2==1){ x+=c/2+r/2+1; } } System.out.println(x-1); }}