using static System.Math;
using System;

public class Hello
{
    public static double a, b, c;
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        a = double.Parse(line[0]);
        b = double.Parse(line[1]);
        c = double.Parse(line[2]);
        var s = (a + b + c) / 2d;
        Console.WriteLine(Sqrt(s * (s - a) * (s - b) * (s - c)) / 4d);
    }
}