public void Return_zero_when_input_is_an_empty_string()
        {
            var result = RpnCalculator.Calculate("");

            Check.That(result).IsZero();
        }
        public void Return_retsult_of_multiple_operations_when_input_contains_multiple_operations(string input, int expected)
        {
            var result = RpnCalculator.Calculate(input);

            Check.That(result).IsEqualTo(expected);
        }
        public void Return_difference_between_numbers_when_input_contains_two_numbers_and_minus_sign(string input, int expected)
        {
            var result = RpnCalculator.Calculate(input);

            Check.That(result).IsEqualTo(expected);
        }
        public void Return_quotient_of_numbers_when_input_contains_two_numbers_and_division_sign(string input, int expected)
        {
            var result = RpnCalculator.Calculate(input);

            Check.That(result).IsEqualTo(expected);
        }
        public void Return_number_when_input_is_a_single_number(string input, int expected)
        {
            var result = RpnCalculator.Calculate(input);

            Check.That(result).IsEqualTo(expected);
        }