﻿namespace topCoder
{
	using System;
	using System.Collections.Generic;

	class p1
	{
		public void foo()
		{
			string[] sp = Console.ReadLine().Split(' ');
			int n = int.Parse(sp[0]);
			int m = int.Parse(sp[1]);
			Console.WriteLine((n * m) / 2);
		}
	}
}
