You have been given an arithmetic expression in the type of a string:
'20 + 7 * 2 - 6'
Create a function that can take this or a similar string as input and return the result of the calculation.
Here are some examples:
'2 + 2' => 4 '7 * 2 + 3' => 17 '(7 - 5) * 20 - 3' => 27