$2\mathrm{.}23$ LAB: Fibonacci sequence

The Fibonacci sequence begins with $0$ and then $1$ follows. All subsequent values are the sum of the previous two, ex: $0,1,1,2,3,5,8,13.$ Complete the fibonacci$()$ function, which has an index, n $($starting at $0),$ as a parameter and returns the nth value in the sequence. Any negative index values should return $-1.$

Ex: If the input is:

$7$

the output is:

fibonacci$(7)$ is $13$