Fibonacci

Returns the desired Fibonacci number. Fibonacci numbers are the numbers in the Fibonacci sequence, characterized by the fact that every number after the first two is the sum of the two preceding ones: F left parenthesis n right parenthesis equals F left parenthesis n minus 1 right parenthesis plus F left parenthesis n minus 2 right parenthesis. The first two numbers are defined as 1: F left parenthesis 1 right parenthesis equals 1 semicolon F left parenthesis 2 right parenthesis equals 1

If n is negative, the Fibonacci numbers are defined as F left parenthesis n minus 2 right parenthesis equals F left parenthesis n right parenthesis minus F left parenthesis n minus 1 right parenthesis

fibonacci(Integer)

Given an integer n, returns the n-th Fibonacci number: F left parenthesis n right parenthesis.