Fibonacci Sequence

The Fibonacci Sequence is computed as f(n) = f(n-1) + f(n-2). We must define two base case values: f(0) = 0 and f(1) = 1.

Thus, the first values of this sequence are: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, etc.

Your task is to write a function fib which takes a single number n and returns f(n) as defined above by the Fibonacci Sequence.