For this task you are going to implement a very simple calculater using a stack.
To simplify this we are ignoring precedence and use post-fix operators (sometimes referred to as reverse polish notation). Post-fix operators are just like your normal infix ones. You just write them
at the end of the two operands. For example 3 + 4 becomes 3 4 +. This makes parsing way easier.
And because we are ignoring precedence, the term 3 4 5 + * is equal to 35.

The concept of a stack based calculator is pretty simple. Numbers simply get pushed on the stack and
operators pop the number of operands of the stack, calculate the result and push it back on.
At the end you can just pop of the result.

Because the parsing and the calculator would be a bit much for one task, we already implemented the parsing. Your task is to fill in the rest of the calculator.