loop do
    m, f, r = gets.split.map(&:to_i)
    break if m == -1 && f == -1 && r == -1
    if m < 0 || f < 0 || m + f < 30
        puts "F"
    elsif m + f >= 80
        puts "A"
    elsif m + f >= 65
        puts "B" 
    elsif m + f >= 50 || r >= 50
        puts "C"
    else
        puts "D"
    end
end
