<s>[INST] The base case of a recursive algorithm for computing the nth Fibonacci number, F(n), is F(0) = 0 and F(1) = 1. Assuming that F(n) has already been computed for all values of n up to k, where k >= 1, we can compute F(k + 1) as follows:

F(k + 1) = F(k) + F(k - 1)

Write a recursive function named `fibonacci` that takes an integer n as input and returns the nth Fibonacci number.[/INST]