namespace Ogorod
{
    using System;
    class Program
    {
        static void Main()
        {
            string[] s = Console.ReadLine().Split(' ');
            double a = double.Parse(s[0]);
            double r = double.Parse(s[1]);
            double kozel=0;

            if ( 2*r <= a ) kozel = Math.PI * r * r;
            if (2 * a * a <= 4 * r * r) kozel = a * a;
            if((2*r > a) && (2 * a * a > 4 * r * r))
            {
                double sektor = r * r * Math.Acos((a * a) / (2 * (r * r)) - 1) / 2;
                double segment = sektor - (a / 2) * Math.Sqrt(r * r - (a * a / 4));
                kozel = Math.PI * r * r - 4 * segment;
            }
            Console.WriteLine("{0:F3}",kozel);
        }
    }
}

Instead of
Console.WriteLine("{0:F3}",kozel);
you should use
Console.WriteLine(kozel.ToString("F3").Replace(',', '.'));
or set locale to en-US.

Edited by author 16.10.2008 04:28

Break on a first test:
66.xxx instead of 95.xxx

Locale en-US is default in Timus Checking System (RTFM).