gistfile1.rb

#!/usr/bin/env ruby
# coding: utf-8

sum = 0
num = 0
STDIN.gets #discard
STDIN.gets.strip.each_byte do |ch|
  sum += 69 - ch if ch < 69
  num += 1
end

score = sum.to_f / num.to_f
#p score
#p sum.to_f
#p num.to_f
puts score