# --> Base method
def sum(rng)
  x = rng.exclude_end? ? rng.last-1 : rng.last
  return ((x-rng.first+1)*(rng.first + x)/2.0).to_i
end

# --> Range class extension
class Range
  def sum
    x = exclude_end? ? last-1 : last
    return ((x-first+1)*(first + x)/2.0).to_i
  end
end