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

namespace Atcoder
{
    public class Program
    {
        static void Main(string[] args)
        {
            var r = Console.ReadLine().Split(' ');
            var H = r[0].ToDecimal();
            var W = r[1].ToDecimal();
            var a = Math.Floor((H * W)/2);
            var ans = a + ((H * W) % 2 == 0 ? 0 : 1);
            Console.WriteLine(ans);
            Console.ReadLine();
        }
    }
}

public static class Extentions
{
    public static int ToInt(this object obj)
    {
        return int.Parse(obj.ToString());
    }

    public static decimal ToDecimal(this object obj)
    {
        return decimal.Parse(obj.ToString());
    }
}

