while line=gets
(m,f,r)=line.split().map(&:to_i)
  break if m==-1&&f==-1&&r==-1
  m=f=0 if m==-1 || f==-1
  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