private static void ThrowExceptionIfCurrencyIsNotValid(string currency)
        {
            if (currency.Length != 3)
            {
                throw new ArgumentException(" ", nameof(currency));
            }

            for (int i = 0; i < currency.Length; i++)
            {
                if (!char.IsLetter(currency[i]))
                {
                    throw new ArgumentException(" ", nameof(currency));
                }
            }
        }