atcoder3_A

gpa_cal.rb

length = gets.to_i
str = gets

result = 0.0
i = 0

str.each_byte do |s|
  case s
  when 65 # A
    result = result + 4.0
  when 66 # B
    result = result + 3.0
  when 67 # C
    result = result + 2.0
  when 68 # D
    result = result + 1.0
  end
end

puts result / length
puts