Hi, here's your problem today. This problem was recently asked by Facebook:

Given an expression (as a list) in reverse polish notation, evaluate the expression. Reverse polish notation is where the numbers come before the operand. Note that there can be the 4 operands '+', '-', '*', '/'. You can also assume the expression will be well formed.

Example

```
Input: [1, 2, 3, '+', 2, '*', '-']
Output: -9
```

The equivalent expression of the above reverse polish notation would be (1 - ((2 + 3) * 2)).

Here's some starter code:

```
def reverse_polish_notation(expr):
# Fill this in.
# 1 - (2 + 3) * 2
print(reverse_polish_notation([1, 2, 3, '+', 2, '*', '-']))
# -9
```