Fibonacci sequence and Fibonacci numbers are extremely famous. The sequence begins with 0, 1 and each subsequent number is the sum of the two numbers before it. Thus fib(0) = 0, fib(1) = 1, fib(n) = fib(n-1) + fib(n-2).

Define a function that when passed an integer, returns the Fibonacci number at that index.

Note: the difficulty is in solving this problem such that you DON'T call the function two times (during each call).

You will most likely need to add a helper function.