using System;
using System.Linq;

class Program84
{
    public static void Main(string[] args)
    {
        var line = Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
        if (line[0] != line[1])
        {
            var mul = line[0]*line[1];
            if (mul%2 == 1)
            {
                Console.WriteLine(line[0]*line[1]/2);
                return;
            }
            Console.WriteLine(line[0] * line[1] / 2-1);
            return;
        }
        if (line[0]%2 == 1)
        {
            Console.WriteLine(line[0] * line[1] / 4);
            return;
        }
        Console.WriteLine(line[0] * line[1] / 4-1);
    }
}