The fibonacci sequence is: 1, 1, 2, 3, 5, 8, 13, 21, and so on, where the first two numbers are 1 and 1, and then each of the remaining numbers is computed by adding the previous two numbers. For example, the 8th fibonacci number is 21 because it is the sum of 8 and 13. Implement a method that returns the nth fibonacci number.