using System;

​

namespace Ceva

{

    public class Program

    {

        public static void Main()

        {

            MathProblems m = new MathProblems();

            m.Calculate("15 + 2+6+2+7", Add);

            m.Calculate("3 - 1  3", Subtract);

        }

    static int Add(string calculation) //Recursive

        {

            char symbol = '+';

            int index = calculation.IndexOf(symbol);

            int a = 0;

            int b = 0;

            int result = 0;

            if (index >= 0)

            {

                string aStr = calculation.Substring(0, index);

                string bStr = calculation.Substring(index + 1);

                if (!int.TryParse(bStr, out b))

                {

                    b = Add(bStr);

                }

​

                a = int.Parse(aStr);

            }

            result = a + b;

            return result;

        }

    static int Subtract(string calculation)

        {

            char symbol = '-';

            int index = calculation.IndexOf(symbol);

            int a = 0;

            int b = 0;

            int result = 0;

            if (index >= 0)

            {

                string aStr = calculation.Substring(0, index);

                string bStr = calculation.Substring(index + 1);

                if (!int.TryParse(bStr, out b))

                {

                    b = Subtract(bStr);

                }

​

                a = int.Parse(aStr);

            }

​

            result = a - b;

            return result;

        }

​

​

    }

    public class MathProblems

    {

        public delegate int CalculateDel(string str);

        public void Calculate(string requirement, CalculateDel type)

        {

            var req = RemoveSpace(requirement);

            if(type != null)

            {

                int result = type.Invoke(req);

                Console.WriteLine( req + "= " + result);

            }

            else 

                Console.WriteLine("Invalid");

        }

        static string RemoveSpace(string initial)

        {

            return initial.Replace(" ", string.Empty);

        }

    }

}