using System;

class Program
{
    static void Main()
    {
        var tmp = Console.ReadLine().Split();
        long r = long.Parse(tmp[0]);
        long c = long.Parse(tmp[1]);

        long ans = 0;

        if (r == c)
        {
            if (r % 2 == 0) ans = r * r / 4;
            else ans = (r * r + 3) / 4;
        }
        else
        {
            if (r * c % 2 == 0) ans = r * c / 2;
            else ans = (r * c + 1) / 2;
        }

        ans--;
        Console.WriteLine(ans);
    }
}