$3\mathrm{.}9$ LAB: Fibonacci sequence $($recursion$)$

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