int result = 0;
            while (true)
            {
                Console.Clear();
                Console.WriteLine("1.Calculator");
                Console.WriteLine("2.Hello");
                Console.WriteLine("0.Exit");
                Console.Write("Choose option: ");
                int menu = int.Parse(Console.ReadLine());
                if (menu == 0)
                {
                    break;
                }
                if (menu == 1)
                {
                    Console.Clear();
                    Console.WriteLine("──────Calculator──────");
                    Console.WriteLine("1.Sum");
                    Console.WriteLine("2.Divide");
                    Console.WriteLine("3.Multiply");
                    Console.WriteLine("4.Subtract");
                    Console.Write("Choose: ");

                    menu = int.Parse(Console.ReadLine());
                    Console.Write("Number 1: ");
                    int a = int.Parse(Console.ReadLine());
                    Console.Write("Number 2: ");
                    int b = int.Parse(Console.ReadLine());

                    if (menu == 1)
                    {
                        result = a + b;
                    }
                    else if (menu == 2)
                    {
                        result = a - b;
                    }
                    else if (menu == 3)
                    {
                        result = a * b;
                    }
                    else if (menu == 4)
                    {
                        result = a / b;
                    }
                    Console.WriteLine("Result = " + result);
                    Console.WriteLine("0.Back");
                    menu = int.Parse(Console.ReadLine());
                    while (menu != 0)
                    {
                        Console.WriteLine("Invalid command!");
                        menu = int.Parse(Console.ReadLine());
                    }
                }
            }
        }