// Check if the EGN is valid
        private static string IsValid(long input)
        {
            long lastNumber     = input % 10;
            long numbersToCheck = input / 10;

            var arr        = digitArr(numbersToCheck);
            var arrsResult = controlNumberCalculation(arr);

            var divideByEleven = (long)arrsResult / 11;
            var controlNumber  = (long)arrsResult - divideByEleven * 11;

            if (controlNumber >= 10)
            {
                controlNumber = 0;
            }

            if (controlNumber == lastNumber)
            {
                return(ValidationService.validEgn());
            }
            else
            {
                return(ErrorService.invalidEgnError());
            }
        }
        private static string egnAuthorization(string egn)
        {
            long input;

            bool validInput = long.TryParse(egn, out input);

            if (input != 0)
            {
                var inputArray = digitArr(input);
                if (inputArray.Length < 10)
                {
                    return(ErrorService.lessThanTenDigitsError());
                }

                if (inputArray.Length > 10)
                {
                    return(ErrorService.moreThanTenDigitsError());
                }
            }
            else
            {
                return(ErrorService.invalidInputError());
            }

            return(IsValid(input));
        }