The Fibonacci sequence is a somewhat fundamental mathematical/natural
sequence whose core property is that every Fibonacci number is equal to the sum
of the previous two numbers.
Fib(n) = Fib(n-1) + Fib(n-2) and
Fib(0) = Fib(1) = 1.
This results in a sequence like 1, 1, 2, 3, 5, 8, 13, 21...
Write a function which takes one parameter, an integer "n" that
returns the n-th number in the Fibonacci sequence.