while line = gets
  m, f, r = line.chomp.split(' ').map(&:to_i)
  break if m == -1 && f == -1 && r == -1
  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