while (m,f,r = gets.chomp.split.map(&:to_i)) != [-1, -1, -1]
  if m == -1 || f == -1
    puts "F"
  else
    case m + f
    when 80..100
      puts "A"
    when 65..80
      puts "B"
    when 50..65
      puts "C"
    when 30..50
      puts r >= 50 ? "C" : "D"
    else
      puts "F"
    end
  end
end