Калькулятор по опз

        static int Calculating(string opz)
        {
            string OPZ = opz.ToString();
            int result = 0;
            Stack<int> stack = new Stack<int>();
            for (int i = 0; i < OPZ.Length; i++)
            {

                if (Char.IsDigit(OPZ[i]))
                {
                    string str = string.Empty;
                    while (!Mest(OPZ[i].ToString()) && !Oper(OPZ[i]))
                    {
                        str = str + OPZ[i];
                        i++;
                        if (i == OPZ.Length)
                            break;
                    }
                    stack.Push(int.Parse(str));
                    i--;
                }
                else if (Char.IsLetter(OPZ[i]))
                {
                    string str = string.Empty;
                    while (!Mest(OPZ[i].ToString()) && !Oper(OPZ[i]))
                    {
                        Console.WriteLine("Введите {0}", OPZ[i]);
                        char item = char.Parse(Console.ReadLine());
                        str = str + item;
                        i++;
                        if (i == OPZ.Length)
                        {
                            break;
                        }
                    }
                    stack.Push(int.Parse(str));
                    i--;
                }
                else if (Oper(OPZ[i]))
                {
                    int a = stack.Pop();
                    int b = stack.Pop();
                    switch (OPZ[i])
                    {
                        case '+': result = b + a; break;
                        case '-': result = b - a; break;
                        case '*': result = b * a; break;
                        case '/': result = b / a; break;
                        case '%': result = b % a; break;
                        case '|': result = b | a; break;
                        case '&': result = b & a; break;
                        case '^': result = int.Parse(Math.Pow(int.Parse(b.ToString()), int.Parse(a.ToString())).ToString()); break;
                    }
                    stack.Push(result);
                }
            }
            return stack.Peek();
        }