using System;

​

public class Program

{

    public static void Main()

    {

        var a = double.Parse(Console.ReadLine());

        var h = double.Parse(Console.ReadLine());

        Area(a,h);

    }

    static void Area(double a, double h)

    {

        var formul = a*h/2;

        Console.WriteLine("{0:f2}", formul);

    }

}