public void AddingNegativeNumbersThows()
        {
            var stringCalculator = new StringCalculatorOne();

            Assert.Throws<ArithmeticException>(()=>stringCalculator.Add("-1"));
        }
        public void StringCalculatorAddsString(string input, int output)
        {
            var stringCalculator = new StringCalculatorOne();

            Assert.That( stringCalculator.Add(input), Is.EqualTo(output));
        }