能横着放就横着放，放不下就竖着放

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

namespace _50A
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] arr = Console.ReadLine().Split().Select(i => int.Parse(i)).ToArray();
            int w_horizon = arr[0] / 2;
            int h_horizon = arr[1];
            int h_vertical = arr[1] / 2;
            Console.WriteLine(w_horizon * h_horizon + (arr[0] % 2 == 0 ? 0 : h_vertical));
        }
    }
}