using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace 練習１
{
    class Program
    {
        static void Main(string[] args)
        {
            var line = Console.ReadLine().Split(' ');
            var lines = new decimal[2];
            for (var i = 0; i < line.Length; i++)
            {
                if(lines[i]>0)
                 lines[i] = decimal.Parse(line[i]);
            }
            var count = lines[0] * lines[1];
            var counts = Math.Ceiling(count/2);
            
            if(counts>0)
                Console.WriteLine(counts);
            
        }
    }
}