I love Fibonacci numbers in general, but I must admit I love some more than others. 

I would like for you to write me a function that when given a number (n)  returns the n-th number in the Fibonacci Sequence.

For example:

```python
   nth_fib(4) == 2
```

Because 2 is the 4th number in the Fibonacci Sequence.

For reference, the first two numbers in the Fibonacci sequence are 0 and 1, and each subsequent number is the sum of the previous two.