loop {
  sum = 0
  m_score,f_score,r_score = gets.split.map(&:to_i)
  break if m_score == -1 && f_score == -1 && r_score == -1
  sum = m_score + f_score

  if m_score == -1 || f_score == -1
    puts "F" 
  else
    if sum >= 80
      puts "A"
    elsif sum < 80 && sum >= 65
      puts "B"
    elsif sum < 65 && sum >= 50
      puts "C"
    elsif sum < 50 && sum >= 30
      if r_score >= 50
        puts "C"
      else
        puts "D"
      end
    else 
      puts "F"
    end
  end
}

