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/2)*(r/2);
		if(r%2==1){
			x+=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);
	  
  }}