$6\mathrm{.}24$ 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 $-1.$

the issue is the input is $20$ and the expected output is $6765$ this is zybooks computing Comp $101$