## rpn

### Instructions

Write a **program** which takes a `string` containing an equation written in `Reverse Polish Notation` (RPN). It should evaluate the equation, and print the result on the standard output followed by a newline (`'\n'`).

`Reverse Polish Notation` is a mathematical notation in which every operator follows all of its operands. In RPN, every operator encountered evaluates the previous 2 operands, and the result of this operation then becomes the first of the two operands for the subsequent operator. Operands and operators must be spaced by at least one space.

- If the `string` is not valid, or if there is not exactly one argument, `Error` must be printed on the standard output followed by a newline.
- If the `string` has extra spaces it is still considered valid.

The following operators must be implemented : `+`, `-`, `*`, `/`, and `%`.


All the given operands must fit in a `i64`.

Examples of formulas converted in RPN:

3 + 4 >> 3 4 +

((1 \* 2) \* 3) - 4 >> 1 2 \* 3 \* 4 - or 3 1 2 \* \* 4 -

50 \* (5 - (10 / 9)) >> 5 10 9 / - 50 \*

Here is how to evaluate a formula in RPN:

```console
1 2 * 3 * 4 -
2 3 * 4 -
6 4 -
2
```

Or:

```console
3 1 2 * * 4 -
3 2 * 4 -
6 4 -
2
```

For receiving arguments from the command line you should use something like:

```rust
    fn main() {
        let args: Vec<String> = std::env::args().collect();

        rpn(&args[1]);
    }

```

### Usage

```console
$ cargo run "1 2 * 3 * 4 +"
10
$ cargo run "1 2 3 4 +"
Error
$ cargo run ""
Error
$ cargo run "     1      3 * 2 -"
1
$ cargo run "     1      3 * ksd 2 -"
Error
```