static void Main(string[] args)
        {
            Converter converter = new Converter(28, 30);

            Console.WriteLine(converter.EURtoUAH(30));
            Console.WriteLine(converter.UAHtoEUR(1000));
            Console.WriteLine(converter.UAHtoUSD(2800));
            Console.WriteLine(converter.USDtoUAH(500));
            // bad tests! (throw exception for negative values) !!!!
            Converter badConverter = new Converter(-28, -30);

            Console.WriteLine(converter.USDtoUAH(-123));
        }