Find the a number coprime to the input

coprime.rb

#!/usr/bin/env ruby


## Initialization and parameter checking:

def usage
  puts "Usage: ./coprime.rb integer\n"
  Process.exit(1)
end


if ARGV.length != 1
  puts "Error: wrong number of parameters."
  usage
end

## Originally, I wrote this to just do GCD, which is why there is a loop here.
## I'm too lazy to change it though.
ARGV.each do |param|
  if param.to_i == 0
    puts "Error: One or more parameters was not an integer, or was zero."
    usage
  end
end

## We can be somewhat sure that we've been supplied one integer at this point.
## So assign the value to a variable.
input = ARGV[0].to_i
## Will be what we increment from to find the coprime.
coprime_seed = 3 # Starting at 3 because why not?

## Recursive Euclidean algorithm taken from: 
## http://en.wikipedia.org/wiki/Euclidean_algorithm
def gcd(a,b)
  ## For Debugging: 
  # puts "gcd(#{a},#{b})"
  if b == 0
    return a
  else
    return gcd(b, a%b)
  end
end

## Two integers are defined as coprime when their greatest common divisor is
## equal to one. Therefore, it follows that if we increment from one and test
## the gcd on each loop, we will eventually find a number coprime to our input.
def find_coprime(a,z)
  if gcd(a,z) == 1
    return z
  end
  find_coprime(a, z + 1)
end

## Main:
puts "Valid coprime of #{input} is: #{find_coprime(input,coprime_seed)}."