using System;

public class Program

{

    public static void Main()

    {

        Console.Write("a = ");

        int a = int.Parse(Console.ReadLine());

        Console.Write("ha = ");

        int ha = int.Parse(Console.ReadLine());

        int s = 0;

            s = a*ha/2;

        Console.WriteLine("S = " + s);

    }

}