using System;

namespace Calculator{
    class MainClass{
        public static void Main (string[] args){
            while (true) {
                int choice;
                string inputNumber;
                double outputNumber;
                double sum = 0;
                bool cont = true;
                bool firstNumber = true;

                Console.WriteLine ("Welcome to my calculator. \nWhich mathematical arithmatic operation do you want to use?");
                Console.WriteLine ("1: Addition (+) \n2: Subtraction (-) \n3: Multiplication (*) \n4: Division (/) \n5: Multiple (+, -, *, /)");
                Console.Write ("\nI want to calculate with: ");
                choice = Convert.ToInt32 (Console.ReadLine ());

                if (choice == 1) {
                    Console.Clear ();
                    Console.WriteLine ("Input the numbers you want to add. End with '='\n");

                    while (cont) {
                        Console.Write ("Input: \t");
                        inputNumber = Console.ReadLine ();
                        cont = Double.TryParse (inputNumber, out outputNumber);

                        if (cont) {
                            sum = Convert.ToDouble (inputNumber) + sum;
                            Console.WriteLine ("+");
                        } else {
                            if (inputNumber == "=") {
                                Console.WriteLine ("Sum = " + sum);
                                Console.WriteLine ("\nPress a key to continue");
                                sum = 0;
                            } else {
                                Console.Clear ();
                                Console.WriteLine ("Error 150: This is not a valid number. Please contact your administrator.");
                                Environment.Exit (0);
                            }
                        }
                    }
                } else if (choice == 2) {//Choice 1 above.
                    Console.Clear ();
                    Console.WriteLine ("Input the numbers you want to substract. End with '='\n");

                    while (cont) {
                        Console.Write ("Input: \t");
                        inputNumber = Console.ReadLine ();
                        cont = Double.TryParse (inputNumber, out outputNumber);

                        if (cont) {
                            if (firstNumber) {
                                sum = Convert.ToDouble (inputNumber);
                                Console.WriteLine ("-");
                                firstNumber = false;
                            } else {
                                sum = sum - Convert.ToDouble (inputNumber);
                                Console.WriteLine ("-");
                            }
                        } else {
                            if (inputNumber == "=") {
                                Console.WriteLine ("Sum = " + sum);
                                Console.WriteLine ("\nPress a key to continue");
                                sum = 0;
                            } else {
                                Console.Clear ();
                                Console.WriteLine ("Error 150: This is not a valid number. Please contact your administrator.");
                                Environment.Exit (0);
                                }
                            }
                    }
                }//Choice 2 above.

                Console.ReadKey ();
                Console.Clear ();

            }
        }
    }
}