ans = []

while true
	args = gets.split(" ").map{|e|e.to_i}
	break if args[0]==-1 && args[1]==-1 && args[2]==-1
	if args[0]==-1 || args[1]==-1
		ans.push "F"
	elsif args[0] + args[1] >= 80
		ans.push "A"
	elsif args[0] + args[1] >= 65
		ans.push "B"
	elsif args[0] + args[1] >= 50
		ans.push "C"
	elsif args[0] + args[1] >= 30
		if args[2] >= 50
			ans.push "C"
		else
			ans.push "D"
		end
	elsif args[0] + args[1] < 30
		ans.push "F"
	end
end

ans.each{|e|
	puts e
}