while true
  m,f,r = gets.split.map {|s| s.to_i}
  if m == -1 && f == -1 && r == -1
    break
  end
  if m == -1 || f == -1
    puts "F"
  elsif m + f >= 80
    puts "A"
  elsif m + f >= 65
    puts "B"
  elsif m + f >= 50
    puts "C"
  elsif m + f >= 30
    if r >= 50
      puts "C"
    else
      puts "D"
    end
  else
    puts "F"
  end
end