using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Text;
using System.Text.RegularExpressions;
using System.Linq;


class Magatro
{
    static long R, C;
    static void Main()
    {
        string[] s = Console.ReadLine().Split(' ');
        R = int.Parse(s[0]);
        C = int.Parse(s[1]);
        if (R == C)
        {
            if (R % 2 == 0)
            {
                Console.WriteLine(R * C / 4-1);
                return;
            }
            else
            {
                Console.WriteLine(R * C / 4 );
                return;
            }
        }
        else
        {
            if (R % 2 == 0 || C % 2 == 0)
            {
                Console.WriteLine(R * C / 2-1);
                return;
            }
            else
            {
                Console.WriteLine(R * C / 2 );
                return;
            }
        }
    }
}