#!/usr/bin/env ruby
# Copyright 2005 Daniel Cer (daniel.cer@cs.colorado.edu)
#
# This work is licensed under the Creative Commons Attribution-NonCommercial-
# ShareAlike License. To view a copy of this license, visit 
# http://creativecommons.org/licenses/by-nc-sa/2.5/ or send a letter to 
# Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 
# 94105, USA.

def fibonacci(n)
  return -1 if n <= 0; 
  fib = prev_fib = 1; n=n-2;
  n.times { next_prev_fib = fib; fib = fib+prev_fib; prev_fib = next_prev_fib }
  fib
end

if (ARGV.length != 1) 
  $stderr.puts "Usage:\n\t#{$0} (n-th fibonacci # to calculate)"
  exit -1
end

n = ARGV[0].to_i
puts "fibonacci(#{n}): #{fibonacci(n)}"