while (m, f, r = gets.split(" ").map(&:to_i)) != [-1, -1, -1]
  if m + f < 30 or m < 0 or f < 0
    puts "F"
  elsif m + f >= 80
    puts "A"
  elsif m + f >= 65
    puts "B"
  elsif m + f >= 50 or r >= 50
    puts "C"
  else
    puts "D"
  end
end