Description

Now you need to consider the reaction of the calculator when users enter expressions in the wrong format. The program only knows numbers, a plus sign, a minus sign, and two commands. It cannot accept all other characters and it is necessary to warn the user about this.

Objectives

• The program should print Invalid expression in cases when the given expression has an invalid format. If a user enters an invalid command, the program must print Unknown command. All messages must be printed without quotes. The program must never throw an exception.

• To handle incorrect input, you should remember that the user input that starts with / is a command, in other situations, it is an expression.

• Like before, /help command should print information about your program. When the command /exit is entered, the program must print Bye! , and then stop.

Examples

The greater-than symbol followed by a space (>) represents the user input.

> 8 + 7 - 4
11
> abc
Invalid expression
> 123+
Invalid expression
> +15
15
> 18 22
Invalid expression
>
> -22
-22
> 22-
Invalid expression
> /go
Unknown command
> /exit
Bye!