loop{ m,f,r=gets.split.map(&:to_i)
  break if m,f,r==[-1,-1,-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
    puts r>=50 ? "C":"D"
  else
    puts "F"
  end
}