using System;
class Program
{
	static void Main(string[] args)
	{
		string[] input = Console.ReadLine().Split(' ');
		int b = int.Parse(input[0]);
		int c = int.Parse(input[1]);
		//出力
		Console.WriteLine(bishop(b,c));
	}
  
  	static int bishop(int x,int y){
     if(x*y == 0) return 0;
     if(x%2 == 0 || y%2 == 0) 
     {
       return x%y+x*y/2; 
     }
     return x%y+(x*y-2)/2;
    }
}