using System;

public class Program

{

    public static double Sabc(double a, double b, double c)

    {return Math.Sqrt((a+b+c)*(a+b-c)*(a-b+c)*(-a+b+c)/8);}

    public static void Main()

    {

        double x = double.Parse(Console.ReadLine());

        double y = double.Parse(Console.ReadLine());

        double z = double.Parse(Console.ReadLine());

        Console.WriteLine(Sabc(x, y, z));

        return;

    }

}