Using C$++,$ please explain in a great amount of detail how the recursive case works. An answer I found says Fibonacci$($n$)=$ Fibonacci$($n $-1)+$ Fibonacci$($n $-2),$ but I do not understand it$.$ 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 $-1.$

Ex: If the input is:

$7$

the output is:

Fibonacci$(7)$ is $13$

Note: Use recursion and DO NOT use any loops.