using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        string expression = "12-2";

        var parsedRawExpression = ParseExpression(expression);

        double result = EvaluateTwoNumbers(parsedRawExpression[0], parsedRawExpression[1], parsedRawExpression[2]);

        Console.WriteLine("Calculation result: {0} = {1}", expression, result);

    }

​

    public static double EvaluateTwoNumbers(string number_1, string number_2, string arithmathicOp)

    {

        double a = double.Parse(number_1);

        double b = double.Parse(number_2);

        if (arithmathicOp == "+")

        {

            return a + b;

        }

        else if (arithmathicOp == "-")

        {

            return a - b;

        }

        else

        {

            throw new ArgumentException("invalid operation");

        }

    }

​

    public static List<string> ParseExpression(string expression)

    {

        List<string> parsedRaw = new List<string>();

        if (string.IsNullOrWhiteSpace(expression))

            throw new ArgumentException("expression can not be empty");

        if (expression.Contains("+"))

        {

            var parsedRawArr = expression.Split('+');

            parsedRaw.Add(parsedRawArr[0]);

            parsedRaw.Add(parsedRawArr[1]);

            parsedRaw.Add("+");

        }

        else if (expression.Contains("-"))

        {

            var parsedRawArr = expression.Split('-');

            parsedRaw.Add(parsedRawArr[0]);

            parsedRaw.Add(parsedRawArr[1]);

            parsedRaw.Add("-");

        }

        else

        {

            throw new ArgumentException("expression is not valid");

        }

​

        return parsedRaw;

    }

}